I've had my license 7 years now - when I did my CBT it was £80 at Centaur I think. CBT expires in 2 years, if you have to take it again I'm sure by that point you'll already own a 125 and won't pay the rental part of a CBT. However, once you've done your CBT I'm sure you'll be itching to take your test and ditch the "L"oser plates - so the 2 year expiry shouldn't be a problem
Personally I'd recommend the following route:
• get a dog of a 125cc (commuter type, not sporty) - you're bound to either have a little spill or drop it at some point early on, so damage is minimal as there's no fairing to worry about.
• take your test before the CBT expires
• Get your first "proper" bike!
With 125's, there's 3 types:
• basic (commuter type/off-road look-a-likes/"chopper"-likes)
• wannabe-sports (CBR-125, R125 etc)
• 2-stroke sports (RS-125, Mito, TZR125, NSR 125)
Don't bother with CBR-125s/R125s - 4-stroke 125s all make pitiful power, the plastics and badges don't hide that.
2-stroke sports 125s can be a lot of fun to ride, but aren't great around town (absolutely no power at low/mid rev range) and can give you many problems.