Real downhill - AKA downhill courses need an Enduro or preferably a DH bike - talking 160mm to 200mm travel.
You'll want a trail bike, probably anything upto 140mm. I'd go out and try a few. Full suspension will add to the cost, but if you check out when
Halfords do their Boardman discounts, and a British Cycling Membership, you'll be able to get the Team FS for less than £1000. I got a Pro FS for £1000 (2014 model).
There are loads to chose from, just depends if you'd like rear suspension or not. I went for suspension with my spinal problems, but am still riding my old rigid MTB as well.