In reply to yh001:
The Project in Poole rents out pads as far as I know, they're near Swanage and Portland. Not sure about around Southern Sandstone as there isn't too much actual bouldering, most of it is quite tall.
I think there are fewer rental places than Font etc because people are more likely to go on holiday to Font by plane than they are to fly to the UK to go to Portland, and therefore are more likely to want to rent a pad than deal with airline baggage restrictions.