With a scooter it all depends on the types of road you will be using . A 125cc will be great around town and in most urban areas. But on open A roads and dual carriageways they are at a disadvantage in that their top speed will be around 50 and most other traffic will be going faster than this. I wouldn't advise you using one on the motorway either for the same reason.
As for the make of scoot the only ones I would advise against are the very cheap Chinese ones as the build quality can be poor and won't see you through a British winter.
Have a look here as this site lists most scooters sold in the UK today.
Sym are a cheaper version of Yamaha and use the same engine and ancillaries so are usually quite reliable.
Peugeot is one of the biggest selling scooter mfrs. around .
If you like the retro look then Piaggio ET2 and 4 may be just up your street, but are generally a bit slower as they have steel bodywork and are a bit heavier than a modern plastic one.
On the whole as long as you keep up with the servicing most modern scoots are very reliable. Service intervals are around 1500 miles / 2500km depending on the make and a basic service will be �50 - �60.
It's when you start to tune them that things can go wrong as you're putting more strain on the quite small motor.
This is a very good mag to read if you want to gen up a bit before buying
http://www.twistngo.com/reviews/index.htm