Ok well I do try not to preach

my self I would say don't shy away from hydraulic disks they really are great. What I find is it has been a shift over the last 5 years more than anything els in that people like the big nobly tires and the rounded rims aswell as disks of some sort being a must. E.g you can't even look at bikes in tesco that arnt offering some form of disk brake system and trigger shifters it's all a marketing and captive audience trick.
Anyway for the money you have available around £450-£550 you can buy a brilliant bike. You will probably find like most of us cyclists once you start you can't stop so really you want a reliable bomb proof starting bike capable of a little rugged terrain not to mention hitting a good pace on the roads to and from tracks. I would recommend spending some time looking into maybe three or four similar bikes and go to a shop have a go on them see how they feel to you. I started on a specialized hard rock about 7 years ago and everyone from family to friends that ask me what bike to buy I point them in the same direction. Not one has ever wished they had bought something els. You can pick them up from a year old on line (gumtree/
eBay/pinkbike) for as little as £250
You won't be disappointed. Hope that helps and if you need anything els let me know pm me if you like