Actually, no, I mis-read that. It's power not size that counts, but they are just as sticky on power and it is quite pedantic.

The UK basic scheme is that you have to get a provisional licence (and insurance) and do a thing called the CBT before you are allowed to ride anything. Then mostly you are allowed to ride a bike of up to 125 cc with power output up to 11 kw (14.75 bhp) with "L" plates and subject to learner restrictions (no pillions, no motorways, etc). Then you take the theory test and the practical test. If you pass then for the next two years you are restricted to 25kw (33 bhp). THe REs are 28 bhp so would be permissible.

There are two other schemes - the accelerated access scheme and the direct access scheme. They are only available if you are over 21.

Direct access - do CBT and theory test then you may ride (with L-plates and other learner restrictions - and ALSO accompanied at all times by an approved instructor on another bike and in radio contact AND if you wear fluorescent or reflective clothing - any size bike while you practise. Then do your practical test on a bike with MORE than 35kw (47 bhp). If you pass you can ride any size bike.

Accelerated access - first get your basic licence. You can ride big bikes (over 25kw) with L-plates and other learner restrictions - and ALSO accompanied at all times by an approved instructor on another bike and in radio contact AND if you wear fluorescent or reflective clothing - any size bike while you practise for a further test that must be taken on a bike with over 35 kw. If you pass you can ride any size bike (and if you fail you don't use your licence to ride bikes of up to 25 kw.

The 25 kw rule also restricts you to bikes with a power/weight ratio not exceeding 0.16 kW/kg.

If you have (a full UK car licence fit counts as a bike provisional licence but you still have to do a CBT and (I think) theory test before riding anything.

All bikes over 3 years old (from date of manufacture) have to have an MoT certificate of roadworthiness, and of course all bikes have to carry road tax.