At some a/g airfields thay will contact a near by ATSU and obtain a clearance for you from them. They will then pass the clearance onto you 'London clears G-OBOY to enter controlled airspace at DCS climbing FL270 Squawk 5762 contact 119.87 passing 6,000'.
Just ask the a/g operator if he is able to obtain clearance for you otherwise open your flight plan when airbourne and hope you don't have a slot!!
------------------
AP