Joined a gym

if you live vaguely near the sea, get a wet suit (you only need a shortie - £25 from Tesco) and use that. You can swim as far as you like and the chill of the water increases your metabolic rate.

IME a 2.5mm thick shortie is fine for swimming (hard) at anything down to about 10C. A full suit is fine for down to 6C, and both together down to around 4C. Just remember to take a thermos for when you get out..

Nah, 80miles to the coast. It may be a nice day out for cyclists but for an evening swim around 7-9pm after a day at work it's no use.

Could swim in the Trent. It's way cleaner than it was back in the mid

How about a local reservoir? There are lots of signs telling you that you'll die, but they're nearly always warmer than the sea, and people seem to manage that ok. Swimming inside seems cramped and strange after you've got used to doing it outside, and paying for the pleasure seems even stranger.

Isn't the problem that the water below the surface can be much colder - the sea gets stirred up. But if you're ready for that, seems eminently sensible.