I have a mix of 70s/80s British and Japanese bikes in various project states
1984 GPz900R - on the road
1977 CB750K7 - running
1974 CB750K2 - rebuild in progress
1971 Triumph TR6/T140 bitsa - running
1974 Norton Commando Mk11a - completely and utterly in bits
All are a hoot to work on and ride (apart from the Norton - haven't tried that yet)
Avoid all the Chinese lookalikes for the time being - their QC is simply rubbish, which is odd for a nation that prides itself on attention to detail in so many other areas.
A few years ago a local dealer had some for sale and we talked generally about them, during which he said he'd be unlikely to work on any of them due to crap spares, unreliability, and low value. In short he said they can be OK when new for a while, but treat them as disposable items!
Stick with the Bantam or CG125 ideas - cheap and plentiful supplies, spares, and knowledgeable internet mechanics
Its unfortunate that some small capacity classic Jap stuff is going for silly money nowadays - I've seen 70s 125s sell for in excess of £2,000
Regarding tests/legislation - best contact your local training centre - the regs have changed substantially over the years and once you pass the compulsory CBT you are limited to capacity/horsepower levels, although there can be exceptions to this I think..........
Also - get onto this........ http://www.classicbikeforum.co.uk/
WANTED: Kona Sex One rear brake canti hanger pivot thingy - GOT ONE!
I could be tempted by a 19" - 21" Dave Yates (mountain) frame.................and no I still don't do trail centreshttp://www.corrieyairack.org/index2.html