ebay - Ribble 531 / 753 with full matching groupset - there's been 3 at the size you want in the last 4-6 weeks at around the £250 mark if I recall correctly. A lot of bike for not much money and that's with the ebay 'premium' as well. Gumtree or the for sale section / wanted section on here may prove fruitful as well.
If I'm spending £250, I will buy a modern bike. The Carrerra TDF, various Dawes Giro bikes, and in particular the red carbon forked Triban 3 are probably what I should
be buying, but I have a long history of going for something a bit different. My first ever motorcycle when I was a teen was a bright orange and chrome Zundapp 50cc.
If your looking at entry level triathlons, the last thing you want is to trying to power a heavy bike bookended inbetween a swim & a run - I'd be looking at the weight of the bike - things like the Record Sprint will mean your expending energy just moving the bike never mind riding a decent lick.
If I don't buy something else, I'll be doing a triathlon on a single speed steel commuter with sit up and beg bars. I haven't tried it, but I suspect that a Record Sprint will be a big advantage compared to that.
Edit: I've googled weights, and it seems that 11kg (24lbs) or so is not unreasonable for a steel framed racer. A Triban 3, probably the 'right' bike for me, is about 10kg (22lbs). I'm not sure that's a big difference, surely I can make up some of that by powdering my nose before a race.
An early 90's onwards bike would be the best bet - likely to be 18 gears minimum with brifters and should you ever need to, replacing parts should be simple and fairly inexpensive.
Could you please tell me which bikes I can look for? I know some current bikes to search for. I'm finding lots of classic bikes to search for. But there's a big 90s/00s shaped gap in my knowledge. The current bikes I know of tend to sell for £200 up. The only modern aluminium bikes I know of that are cheaper are some of the Viking bikes. Having bought a Viking single-speed, I'm a bit worried about buying a more mechanically complicated bike from them.
Best of luck, and welcome aboard the good ship Retrobike