Hello and welcome to the forum
You are correct in identifying a hybrid as a suitable for such a task and five hundred quid should get you a very respectable one. Its a fiercely competitive market at that price point so you shouldn't get anything over priced.
Firstly don't believe everything you read on the internet! Hybrids cone in various 'flavours' but just about any would suit the task you want it for, tarmac and trails. Which do you think you'll spend more time on? At the speedier end a bike with 28-32c tyres will be more suited to the road and bikes with wider 35-42c tyres are more suited to off road situations, both will cross over but wider tyres will feel more comfortable on tracks, but a bit slower on the road, but not by much.
I suggest you get down to a good bike shop and try a few, a good one should be able to give you knowledgeable advice. A local shop is always handy should you experience any problems, rather than ordering on the net. They will also be able to find the correct size for you. Any of the major brands are good, Specialized, Giant, Trek, Cannondale, Merida, Jamis etc. Shops will tend to concentrate on two or three, larger stores more.
Good luck. Any other questions just ask.