I am a female living in England. I would like to know whether it is permissible for menstruating women or anyone in a state of major ritual impurity to enter the mosque.

It is permissible for women in a state of menstruation or postnatal bleeding and anyone in a state of major ritual impurity (janabah) to enter the mosque either in passing, to fulfill a need, or to attend educational classes and the like. This is because there is no prohibition mentioned in this respect.
This is the opinion of Ibn Hazm, al-Muzani from the Shafi'is and Dawud. They based their opinion on a number of evidences including what Ibn Hazm reported in his book Al-Muhallā bi'l Athār. He said, “The mother of the believers, ‘Aishah (may God be pleased with her) narrated, ‘There was a black female slave owned by an Arab tribe and they freed her. The girl came to Prophet Muhammad (peace and blessings be upon him) and embraced Islam. She had a tent or a small room with a low roof in the mosque.’”
The above narration illustrates that the woman was permitted a space inside the mosque of the Prophet (peace and blessings be upon him). The Prophet (peace and blessings be upon him) did not prevent or prohibit her from staying in the mosque even though it is natural for females to menstruate.
In another report, Abu Huraira (may God be pleased with him) narrated that the Prophet (peace and blessings be upon him) met him on one of the streets of Medinah while he (Abu Hurairah) was in a state of major ritual impurity. He hurried back and took a ritual bath (ghusl, to remove the state of impurity). The Prophet (peace and blessings be upon him) noticed his absence, and when he came back, the Prophet (peace and blessings be upon him) asked, ‘Where were you, O Abu Huraira?’ He replied, ‘O Messenger of God, you met me while I was in a state of major ritual impurity, and I did not want to sit in your presence until I had performed the purifactory bath.’ The Prophet said, ‘Glory is to God! The believer never becomes impure.’” This hadith shows how the Prophet (peace and blessings be upon him) saw no reason for Abu Huraira's absence from the class of knowledge held inside the mosque due to being in a state of major ritual impurity.
The people of Suffah used to stay overnight inside the mosque in the presence of Prophet Muhammad (peace and blessings be upon him) and they certainly included those who had wet dreams. The Prophet, however, never prohibited them from staying overnight inside his mosque.
On the other hand, some jurists have maintained that it is impermissible for menstruating and postpartum women and anyone in a state of major ritual impurity to enter a mosque except if there is a necessity such as if they were passing by, seeking protection against outside harm etc. Other jurists maintain absolute prohibition for anyone in such a state.
It is necessary to clarify the degree and classification of the hadith upon which the latter group of jurists based their opinion. Abu Dawud, Ibn Majah, al-Bayhaqi and Ibn Khuzaimah reported that ‘Aishah (may God be pleased with her) narrated that Prophet Muhammad (peace and blessings be upon him) said, “The mosque is not permitted for menstruating women or anyone who is in a state of major ritual impurity."

The hadith’s chain of transmission
All the narrators mentioned in the hadith’s chain of transmission are unreliable and their narrations are considered weak. The chain of transmission includes Jassrah Bint Dajjah whose narrations, according to al-Bayhaqi, need to be reconsidered. In his book Al-Jarh wal Ta'deel, Ibn Abu Hatim mentioned that the chain of narration includes Fleit Ibn Khalifa who reported from Jassrah through ‘Aishah, and Fliet is unknown.

The text of the hadith
The entire text of the hadith is considered weak. This is the opinion of Al-Khattabi in Ma'alim al-Sunan, Ibn al-Qayyim in Tahdhib Al-Sunan, al-Nawawi in Al-Majmu' and Ibn Hazm in Al-Muhalla. Ibn Hazm said it is a fabricated disapproved hadith.

Non-Muslims are welcomed inside mosques
One of the things that support the opinion allowing menstruating and postpartum women and anyone in state of major ritual to enter a mosque is the fact that non-Muslims are allowed to enter.
During the Prophet’s lifetime, non-Muslims used to frequently enter the mosque either to embrace Islam, convey a message (to the Prophet), or discuss or debate issues as was the case with the Christian delegation of Najran that stayed in a corner of the Prophet's mosque for days. Likewise, the Christians of Abyssinia used to hold shows inside the mosque of God's Messenger with their spears in the presence of the Prophet (peace and blessings be upon him) and his Companions.
Since non-Muslims are allowed to enter mosques, a fortiori menstruating and postpartum Muslim women and individuals in a state of major ritual impurity who wish to attend classes, study, and teach the Quran. Likewise, if it is permissible for menstruating women and individuals in a state of major ritual impurity to pass through mosques for a certain need such as to seek shelter from heat or get some rest, so it is even more so that they enter for the purpose of seeking and teaching knowledge.
The claim that the reason behind the prohibition of allowing menstruating and postpartum women inside mosques is the fear that they might soil the place where people perform their prayers is no longer applicable at present due to the existence of more dependable feminine hygiene products. Moreover, it is permissible for women to fast, pray and attend religious gatherings during istihadah (bleeding outside regular menses).
‘Aishah (may God be pleased with her) narrated, "One of the wives of Prophet Muhammad (peace and blessings be upon him) practiced i'tikaf (spiritual retreat) with him while she was in a state of istihadah. She would see red blood and would sometimes put a tray beneath her when she offered prayers.”
If the reason behind forbidding menstruating women from entering mosques is fear of leaking blood, it takes the same ruling as the case of istihadah. If women were permitted to enter the Prophet’s mosque during his lifetime, it is illogical for us to ask our women to refrain from our mosques especially with the new hygiene products available to women. Moreover, women at present are in need more than ever of religious knowledge by means of attending gatherings of religious knowledge.
In another hadith, the Prophet (peace and blessings be upon him) encouraged all women to go out and attend the ‘Eid prayer with the aim of allowing them to partake of the blessings and goodness of the day and make remembrance of God. So, women are invited to attend ‘Eid prayers to learn and benefit from the sermon and supplicate to God with other Muslims. This is typically what classes in mosques offer.

The ruling
It is permissible for menstruating and postpartum women and anyone in a state of major ritual impurity to enter mosques, pass by them or attend classes held in them and so on. However, a woman, whether experiencing menstruation, postpartum bleeding or istihadah, must use proper hygiene products to avoid soiling the mosque. If space is limited inside a mosque, it is recommended that they leave at the time of prayers to make space for other women to perform prayers.
And God Almighty knows best.