Several options for this but none of them are going to be as good as outside the city. I live in West London so these are my options…
Richmond park – a lap takes ~ 20minutes on the perimeter road. If you go round anti-clockwise you don’t have to stop at all. It is surprisingly hilly so for steady pace I don’t find it ideal. The traffic is not great but dies out in the evening – loads of people riding.
Richmond park – Couple of hills for sprint intervals on – 3 minutes to 5 minutes up a hill easily possible.
Bushy park further out towards Kingston / Teddington. Has a long flat road thought the middle of it – again useful for intervals – dead end though so can’t again do continuous steady training.
If you have something other than road bike can go all the way along the Thames from Putney to Hampton – almost no road and flat – gravel tracks. Can be a fair few people out and about on it.