What you doing the full test on? Most schools use ER5 or equivalent for A2 tests and Bandit6/Divvy6/SV650 etc. for the full A class DAS. Getting a peaky stroker or laidback cruiser is not going to give you the same handling characteristics as these bikes, so you'd probably be better off on a YBR/CG/EN - basically a simple bread and butter commuter style machine. No, they're not as much fun as a DT, but you're aiming to get the test out of the way so you can progress onto the fun bikes, so suck it up buttercup and bear your apprenticeship like a man.
Remember, the 500 quid 125 you buy now will get you through your test and will still fetch near enough 500 quid from the next learner who buys it off you. Best bet would be a multi-owned CG or similar with a chain like a Portsmouth prossie's knicker elastic and oil like pi$$ - good bargaining points to get a fair old amount knocked off the asking price, but simple and cheap to correct yourself, bearing in mind these engines are designed to be abused by spotty 17 year old oiks so are pretty much bombproof. Get one, spend 50 quid or so servicing it, pass your test and flog it on for probably 100 quid more than it cost you - yes, I speak from experience.
Happy hunting!