A Claud Butler Professional like this
? That was a bargain: just the frameset for £150 would have been cheap.
My first piece of advice would be to ride before you buy: but you already know that.
Second would be to join the local Triathlon club: apart from being able to offer advice on specific bikes, it's likely someone will have one to pass on.
If you are, mechanically speaking, a novice then buying an old bike second hand on a budget is not what I would recommend, especially to compete on even at a modest level.
The current post-Olympics/Bradley Wiggins bike boom combined with the usual pre-Summer upturn, means you've got every monkey and his Uncle dragging the myriad of run-of-the-mill Raleighs and Peugeots out of the shed where they have been languishing unloved and unmaintained for the past twenty or thirty years and sticking the "retro" tag and an inflated price on it.
Interesting you mention the Triban 3: excellent value and highly recommended. Other decent "beginners" bikes to look out for are the Raleigh Airlite 100, Edinburgh Bike Co-Op Revolution Continental, Halford Carrera TDF Ltd and Dawes Giro 300.
The "bitsa" in Leicester could be a hidden gem and absolute steal, but do you know what to look for? "minimal amount of damage to frame" covers a multitude of sins, do you know what to look for? Apart from which, it is a lot smaller than the other bikes you linked to: do you know what size you need?
The Record Sprint: £100 without wheels? Move on.
The Quasar is the "least worst" of the three, but over-priced: if it came from a shop with six months warranty and had been serviced by a qualified mechanic then you might be able to justify the premium, but being "completely checked over and given a full safety check" by some bloke on Ebay means nothing.
With your limited experience it would be sensible to pay the extra that buying from a shop would involve, but £250 for a Carbolite Pug? Only £150 over the odds.