In reply to rhinobiker:
It rather depends on your gear.
If you don't have a rope, then obviously you can't go outside at all unless accompanied by an instructor all day (in which case a bit of trad in the Peak would make sense).
If you don't have any quickdraws, then Portland will be pointless because you generally can't set up top ropes from above. If you do have quickdraws and a rope (plus a sling and screwgate each) then you can do sport climbing on your own; there are good places in Portland but it helps to be selective . If you are a bit crap, like me, then quite a lot of it is too steep :P for example, Dungecroft has 4s and 5s that are short and steep and therefore hard! Whereas the Lost Valley and Neddyfields around the corner are much more pleasant. The Cuttings has some easy routes but the one to do are the ones tucked away around the corner (the upper/lower Bower, and the three routes in the Dell); the easy routes on the main face are super shiny!
If you had trad gear (which I assume you don't), then you can either trad or set up top ropes. I have set up top ropes just using trees, screwgates, slings, some static rope and the one end of my lead rope before I got my new shiny (as yet unused) trad rack, but this relies on knowing that there are suitable trees/slingable boulders. But for short climbs (less than 1/3 rope length plus anchor length) with a convenient bomber tree, this may be useful as all you need is a few screwgates and a car mat or similar to pad the edge. Two tree minimal anchor... tie rope to tree. Run rope over edge, tie fig 8 on a bight, put screwgate (or two) in bight. Run rope back over edge, tie to another tree, possibly clove hitch or rethreaded fig 8, if you have a sling this is easier. Throw middle of rope to ground. Pass end of rope through screwgate(s) and thread down to ground, tie into this end to top-rope. Don't fall off edge at any point! Take great care to avoid rope rubbing on edge - since you are using dynamic rope for the anchor, it will stretch when in use so will rub if not properly padded.
Be prepared to drop grades dramatically if you are anything like me! (climbed a (soft) '6c' top-rope indoors yesterday, never done a 6a outdoors - although probably should be able to) :P