We’ve amended this article based on feedback….
While there is believed to have been at least one earlier mosque opened in London (a room in a house was apparently converted for this purpose in Albert Street, Camden, in 1895), London’s first purpose-built mosque opened in 1926 in Southfields in the city’s south-west.
Known as the Fazl Mosque (Grace Mosque in English), the mosque – which is also known as the London Mosque and is the second oldest in Britain – is said to have cost some £6,223 and is understood to have been designed by TH Mawson and Sons.
Said to have been financed by the donations of Ahmadiyya community in India (the land had been purchased in 1920), the foundation stone was laid in 1924 by Mirza Bashir-ud-Din Mahmud Ahmad, the second Caliph of the Ahmadiyya and leader of the worldwide Ahmadiyya Muslim Community.
The Grade II-listed building was formally opened by Khan Bahadur Sheikh Abdul Qadir, an ex-minister of Punjab Legislative Council, on 4th October, 1926, with about 600 guests.
The mosque can accommodate about 150 people and the first Imam was Maulana Abdul Rahim Dard.
Visitors to the mosque have included King Saud of Saudi Arabia and the Crown Prince Faisal Bin Abdul-Aziz, who visited in 1935. In more recent times, Prince Edward, the Earl of Wessex, and Justin Welby, the Archbishop of Canterbury, have both visited the property.
The fourth caliph, Mirza Tahir Ahmad, lived in an apartment in a separate building on the premises, originally built for the mosque’s imam, after migrating from Pakistan where prohibitions were placed on the Ahmadis, banning them from any public expression of the Islamic faith.
It remained so until his death in 2003 after which the current caliph, Mirza Masroor Ahmad, lived there until relocating to the estate known as Islamabad in Tilford in 2019.
During this period between 1984 and 2019, the mosque at 16 Gressenhall Road served as the de facto headquarters of the Ahmadiyya Muslim Community worldwide.
Exploring London 
Clearly not true I’m afraid.
London’s First known Mosque was 1895 on Albert Street, Camden Town, established by Hajee Mohammed Dollie.
Furthermore, the place you quote as the second mosque in UK is wrong as other Mosques had been established too
Thanks for your comment – of course, we always welcome feedback on the “oldest” as it can be controversial. In this case, our comment of the Fazl Mosque being the oldest purpose-built mosque is based on the official listing of the Fazl Mosque on the Historic England website which in its official entry describes the mosque as the “first purpose-built mosque in London and only the second in Britain” (https://historicengland.org.uk/listing/the-list/list-entry/1454338?section=official-list-entry). Historic England furthermore names the Shah Jahan Mosque in Woking as the the first purpose-built mosque in Britain and northern Europe. (https://historicengland.org.uk/listing/the-list/list-entry/1264438?section=official-list-entry). As you told the Londonist, the Camden mosque was established in 1895 in a room in a house in Albert Street by Haji Mohammed Dollie so it was indeed an older mosque (but not the oldest purpose-built mosque). As far as we can gather, it is no longer used as such (but happy to be corrected on that if you have further information).