Hello buddy, I did my hour building at Thurxton this time last year and found that a great place for the 300nm was to Dunkeswell then down to Lands End (St Just). When you get there spend the day at Sennen beach then fly back, absolute cracker!! That way it would take you through SOU airspace you could then navigate past Bournemouth, Yeovil, Exeter and the Dartmoor danger area then poss do some VOR practice tracking to LND. Depending on where you are doing your CPL/IR could be an idea to try to fly in that direction more during you hour building.
Best of Luck
CABUS