I’m not aware of anyone organising rides that match that description.
There are plenty of road cycling clubs round here who do regular evening and weekend rides. - Islington , Finsbury Park, Muswell Hill, London Phoenix, etc. I’m guessing that’s not quite what she’s after though. Islington probably the most friendly of those.
iBikeLondon organise monthly social rides around central/inner London (Facebook is the best place to check them out). The other obvious one is CTC Sunday rides which start outside town.