Greetings Steve,
The golden answer to this question could earn the respondent a Sainthood
I too would suggest that you stay to an older type 250cc four stroke until you gain experience and know exactly how you wish to progress. If you are going to ride in the Peak District then a large engine and/or fire breathing enduro bike is definitely the wrong way to go. I ride an 'old' 225 Serow which is far more capable than I, height of the foot pegs is the only down side, for greater distance outings I use a Beta Alp 4.0 350cc which has the old Suzuki engine, the pair cost me less than your projected top bracket.
You will need funds for safety clothing, that alone could set you back a few bob depending upon your choice. Think on that at some stage you are going to drop the bike and that it is going to get scratched
I know folk who gave up on trail riding their bright new shiny bike because they could not stand to see it dirty
You need to bear in mind that the bike will need servicing which could get expensive depending upon the marque and if you have to pay for such at a garage. Finding a reliable company can also be difficult so go off local knowledge not shinny showrooms and seat sales talk.
Might I suggest that you keep your money in your back pocket for now and take yourself along to your nearest group to chat with the folks already riding the lanes in your area, one of them might have a 'used' but reliable bike for sale well below your ceiling. Simple uncomplicated engineering is the secret.
All of the groups are listed at;
http://www.trf.org.uk/find-your-regional-group/Manchester; Derbyshire and South Yorkshire; and High Peak and Potteries are all very active in your area and they will all make you most welcome
TTFN
Hugh.