I’ve got a pair of the Decathlon Fast Hiking trousers that were about 40 quid. They aren’t waterproof, but they are very lightwieght (may not actually be a positive – I’m yet to take another tumble on tarmac), dry rather quickly, have two not very deep pockets (useful for stashing things briefly, not for riding with), and have a nice taper from reasonably roomy to slimmer at the ankle which has resulted in zero interactions with the drivetrain. They’ve got a windproof panel on the front also. I pop a pair of tights underneath for these colder conditions. I have been very impressed.
A neighbour of mine swears by the Keela Roadrunners though. If I have 80 quid spare at any point I might give them a go.
Oh, and I wear medium Endura Singletrack knee pads. They fit nicely under the trousers – plenty of wiggle room.