Assuming that the recordist is using discreet recording equipment rather than brandishing a Rycote windshield and a Sound Devices 702 in a bag with EXPENSIVE EQUIPMENT IN HERE! written on it, the following places are worth visiting:
The West End in general with Soho, Covent Garden and Villiers Street/Embankment in particular worth visiting at night.
Brick Lane and Petticoat Lane markets on a Sunday morning. They are both quite near to each other in east London, try to get there by 11am at the latest.
Best echoes are in the long pedestrian tunnel leading from South Kensington tube station.
Speakers Corner in Hyde Park on a Saturday or Sunday.
Whitehall end of St James's Park fairly early in the morning. On the hour the bells of several churches and towers can be heard striking within a few seconds of each other.
Brixton by day, especially the area outside Brixton tube station, the arcades and street markets, Atlantic Road, Railton Road, Electric Avenue.
Great sounds at two museums in Brentford: the Steam Museum (has working stationary steam engines) and the Music Museum, which has player pianos and other mechanical, self-playing instruments.