In reply to DanielGyi:
Local-ish;
North Downs/Surrey Hills. I've done sneaky wild camps in the woods there on occasion.
Chilterns/Thames - some sort of loop over the Chilterns and returning by the river, maybe using Pangbourne as a start/finish.
SOuth Downs - plenty of options, could do station to station easily.
A few hours on the train;
Dorset coast
Black Mountains via Abergavenny.
Peak District
Long weekends;
Anything! I've just done a weekend in Knoydart leaving work near Brighton on Thursday evening, heading up by train and sleeper to Mallaig, ferry in to Knoydart, 3 nights there canoeing and walking, return to Mallaig Monday morning by canoe, train/sleeper to work Tuesday morning. Hauling a folding canoe and camping gear all the way. Straight back into a 3 hour meeting, at which I will admit I was running slightly under power.
Easier versions would include Exmoor/Dartmoor/Yorks Dales/Lakes etc. Snowdonia a bit harder by train if I remember rightly, need bus from coast which was very sporadic.