My advice is a little bit different from u/BigRedS. As a first pair of trousers, you will probably be immensely happy with these ones by Richa. I've worn them for almost a year going from 2C (thermal leggings with the Richa thermal liner) to 25+C in the recent heat wave. What makes them amazing is their fit. They fit my thighs really well, and with the ankle slimming down for a more fashionable look.
The biggest problem with these trousers is the baked-in rain liner, which gets hot above 23-24C. However, the trick I've learned to wearing these in hot weather is to wear a very thin pair of sport leggings. This wicks the sweat away from your legs and keeps your skin from contacting the nylon liner.
You asked:
Have you done much winter riding in those?
The Revit ones that /u/BigRedS linked are more like 3 season hot weather. The Richa ones I linked are 3 season cold weather.
I've seen similar trousers going for around half the cost, is there much difference?
If I compare the Revit with my Richa, here are the differences that are accounting for cost.
Name brand. Rev'It is a bigger brand.
Better zippers. The zippers in the Richa, especially around the crotch are cheap. They've not failed me yet, however.
Slightly better velcro attachment system at the ankle.
Slightly better armour.
Other than that, I don't see much difference. There are some cosmetic differences in the Rev'It trousers, but to be honest, I think those trousers are maybe £20 overpriced. Plus I think that mesh is slightly less versatile in the UK weather.
In looking for trousers, I highly recommend:
3 season cold: I think this is best for UK weather
Rain liner: I prefer overjacket rain gear on my jacket, but I prefer built-in rain liner in my trousers, for the simple reason that it's easy to put on an overjacket on the side of a road...not so easy to put on rain trousers.
Elasticated knees: don't go for ultra cheap stuff that doesn't have it.
Velcro attachment at the ankle: nobody wants that flared look
Thermal liner
Bonus: removable rain liner. Now these are really hard to find, and it's what separates cheaper trousers from expensive trousers.
I personally would rather spend sub-£120 on a pair of trousers that works well 90% of the time, and then another £100 on a pair that will take care of the odd situation (heat wave, for instance).