20 year break, Which Bike?

CB125S was my first bike. If you rode a good amount (say, over 5000 miles/year) and did that for at least a year, I'd say take an MSF course as a refresher (traffic conditions have changed) and get something midsized and that fits you. The following bikes are marketed as Re-entry bikes: Ducati 696, Triumph Bonneville series, Kawasaki 650R, Versys, Vulcan 900, Honda Shadow series, VTX1300 series, Yamaha FZ6, Star 650s, Suzuki Gladius, SV650, GSX650F. Also, pretty much any mid size dual sport is great for taking out onto dirt to practice in low traction environs.

Any thing that is comfortable to you , you have to remember the average 650 made to day is on par of what a 1000cc was 20 years ago. Take a MSF to refresh. And as usual Bart is blowing smoke. Scooters are not what they use to be but if you want a big scooter and they are fun look at Kymco they blow the Burgmans out of the water even their 250 or 500 scooters will run circles around the 650 Burgman. And Kymco is Cheaper. Also Hyosung makes a really nice 650, Both Kymco and Hyosung will be new names to you but they are quality companies not imported Chinese junk. They use to make motors for Honda and Suzuki they just started selling their own. And there is the Harley sportster

You're in the UK, and all you need on the B roads is a 125. The A and M roads are for going places, not for riding.

no offense, but are you an older person?

if you are an older person I would suggest the Suzuki Burgman 400, or 650. yes, its a scooter, but its a scooter that rides and handles like a sport bike, it has the power and acceleration to match a bike twice its cc's, plus it very comfortable, easy to ride, excellent wind protection, you can ride down the road on one smoking a cigarette and hear the birds singing at 70 mph, you can take a cross country trip with one and feel no discomfort, excellent reliability, excellent mileage.

I agree think a Honda Vespa would be right up your alley. I mean i clearly see you going through a midlife crisis and your questioning your hip factor. My suggestion to you would be the Vespa if you bought that thing you would get more tang than an astronaut.