Yes, London’s a great city, something for everyone if you have the money, and the Olympic legacy is going strong. Shame Boston’s knocked that one out.
Cycling
The velopark is fantastic - capital tri do some regular duathlon’s on the road circuit which are usually pretty good, and a wide range of abilities. The park also hosts crits there, not to mention being able to try your hand at track cycling at the velodrome itself.
There’s a bunch of cycling clubs around the London area, although they’re usually area specific - the ‘stronger’ clubs are usually down in South London, but North and East London have some good clubs also. Rapha also have a cycling club, but I can’t vouch for that. British Cycling should help in locating one.
The traffic in London is abysmal, but I felt OK cycling in London. The most trouble I had was with fair weather or newbie-wannabe cyclists basically doing whatever they pleased and pissing everyone else off in the process. I was based in East London so it was fairly easy for me to go out after work from Mar-Oct for a quick 20mi, but otherwise the trainer was a dear friend. Ignore the haters advising on getting a tonne of wet weather gear. It’s not that bad.
Don’t forget the time trialling scene in the UK, with distances from 10mi after work blast to 24 hour run-offs. Whilst it’s anachronistic, it helps if you know someone to show you the ropes about finding an event and getting on the start list. Your local tri or cycling club should be able to help you on this.
Swimming
The Olympic pool is amazing, and yes, you do need to book but it’s usually not a big deal. London Fields lido used to be a favourite of mine. A wide 50m pool, no late night opening, but with a regular coached session with London Fields Tri team (was never a member, but seemed nice enough when I met them on the deck or at races). Hampstead Heath lido (Gospel Oak) also looked nice, but I never managed to organise myself to get there. There’s lots of 25m pools about, as mentioned earlier.
As mentioned earlier, capital tri run regular OW swims, races and aquathons at the West Reservoir, near to Highbury (just up the road from KingsX). There’s also the serp, as mentioned earlier, but if you fancy travelling out further there’s lakes around Heathrow and in Essex. Swimfortri do some coached sessions, open session and races at a variety of locations.
Running
Parkruns, canals, Royal Parks, regents park 10k series, a bunch of other organised races from 5k to marathon (Hyde Park last Fri race was a fave). There’s a tonne of stuff to do.
Events
Similarly there’s lot of triathlons/duathlons - Virgin London, ITU London, Thames Turbo sprint series, capital tri series, Dorney Lake has lots on, amongst others, more around an hour or two away if you can travel. British Tri, Tritalk.co.uk or Tri247.com can help in this regard. The championship qualification races are usually good if you want some competition, but they can rotate around the country.
Clubs
There’s so many nowadays, but off the top of my head, those that haven’t been mentioned - London Fields (central east), Serpentine (central), Thames Turbo (west London), Greenwich tritons (south-east). British Triathlon should be able to help find you clubs, then it’s all about which club fits you best.
Hope you have fun - I really enjoyed my 15 years there.