We are in London with a 7yo and 3yo and it really suits us. We have a short cycle commute (10 mins) which makes a big difference with 2 dcs and school/preschool events like assemblies and sports day - both parents have been able to attend all of them so far. And we never worry about issues with trains or tube strikes.
We have a big park near us which is more interesting than the garden when dcs are a bit older - much more to explore, features like splash areas and climbing, more sociable and frequent events. We also go all over London to the big playgrounds like Diana Memorial, Battersea and Holland Park.
There's a huge variety of events, festivals and workshops in the museums and parks, lots of different soft plays, arts events, concerts, kids theatre and concerts, so we never get bored. A lot of it is interesting for us as adults too (we have just started taking the dcs to West End musicals) so it doesn't feel like a chore to take them.
Plenty of after school activities, and summer activity camps for primary age. Great opportunities for secondary aged kids in the future with world-class institutions and uni links.
Easy to travel out of London for day trips (lots of theme parks close enough for day trips) or going to airports. We never need to book an airport hotel and we usually just get a bus or tube to the airport or Eurostar.
We have no plans to leave, unless we went to live abroad.