A year ago a friend of mine rented an aztec in Elstree, self hire with no instructor for 160 GBP per hour, the aircraft was old but well maintained.
600 GBP sounds like theft to me, after all is not the space shuttle you want to fly!!!!!!, as Richiya said you better check out the prices in Spain or Portugal, you should be able to get it for 200 GBP per hour.
Last infos about renting a piston twin in spain for ME-IR renewal was 300 euros an hour which is around 200 GBP, that was in Madrid.
Good luck