My two favourite places to run are quite different from one another. One is the Peak District National Park - not as challenging as the Brecon Beacons can be, but still tough, yet beautiful. Lots of nice trails etc, and of course if you want to you can run along the Pennine Way.
My other favourite is running through Manchester city centre. I don't know why this is, but I find running past all my favourite shops, pubs and clubs to be very enjoyable. Not to mention the feeling of smug superiority you get when you, the at-peace Zen runner, streak past moronic Yuppies helpless at the traffic lights.