Swimming

Better to get better in a pool first before venturing outside. As @Greenbank the next logical step is an organised lake where there are lifeguards and stuff, plenty around the west side of London but most will have closed for the winter now.

If you're really keen on sea swimming look up your local open water swimming society, usually found on Facebook and there are likely to be crazy regulars there who are super encouraging of new people to the sport and probably swim all winter in skins (no wetsuit).

swimming on your own is really not recommended, between just getting into difficulty, currents or water craft can all go from something minor to a full on disaster very quickly. whistle or no whistle if there's no one watching then it ain't gonna do much good!

Tow floats are good for visibility and a bit of buoyancy, but I would want to need one if you know what I mean, they do all come with whistles but as said before who's attention are you trying to get, and if they heard you blowing away are they going to be able to do anything to help you in time!

Wetsuits are definitely a good idea for beginners both for floatation and warmth when you are first going from a 23 degree pool to 10 degree water it can be a bit of a shock!

Finally, MP3 players in the pool are a bad idea as you can't hear other things around you but are an absolute no no in open water, if you can't hear the boat coming towards you how are you going to get out the way!!