If I was flying from Biggin Hill to Cardiff, I think that I’d probably route to Wisley/OCK VOR at around 2300 ft, then direct to the Marlborough VRP in Wiltshire. Call Farnborough before reaching the Farnborough/Blackbushe/Odiham area (they should be able to tell you whether D133 is active above 1200 ft) and then climb to 3000 ft once past Blackbushe in order to avoid bonging the Aldermaston restricted area (R101). At Newbury, change to Lyneham and ask to cross the Lyneham zone direct from Marlborough to the Portishead VRP on the coast of the Severn. Once past Colerne, change to Filton and ask to cross from Portishead VRP to Cardiff Docks VRP, calling Cardiff when clear of Portishead. Or, if you don’t like the idea of a longish Bristol Channel crossing, cross direct to Wales and then fly down the coast to be at 1500 ft at the Cardiff Docks VRP.
Any other suggestions?