Geoffrey of Monmouth in 'The History of the Kings of Britain' mentions two temples in connection with pre-Roman London - the Temple of Concord and the Temple of Apollo.
The Temple of Apollo is thought to have been where Westminster Abbey is now and there is no doubt that the Abbey was built on an already sacred site, the present abbey was built on the site of an earlier Late Anglo-Saxon church (Edward the Confessor). This in turn replaced an earlier church which legend says was consecrated by Saint Peter the apostle.
By this surburbes Westminster, which some time was more than a mile distant, is conjoined so close unto the City of London that it seemeth a member thereof, whereas it is a City of it selfe, having their peculiar magistrates and priviledges. It was called in times past Thorney of Thornes, but now Westminster of the West situation and the Monastery. Most renowned it is for that Church, the Hall of Justice, and the Kings palace. This Church is famous especially by reason of the Inauguration and Sepulture of the kings of England. Sulcard writeth that there stood sometimes a Temple of Apollo in that place, and that in the daies of Antoninus PiusEmperor of Rome it fell downe with an earth-quake.
Considering the importance of religion it may seem odd, but fashion, in its broadest sense, seems to have had an influence on which temples were built, and when, and who they were dedicated to.
Because of this it ought to be possible to get a fair idea of how these temples would look like from comparing them with other known examples, here are a few -