For your own peace of mind and assuming you rent the same aircraft it may be worth getting a transponder readout check with farnborough to see what accuracy your transponder is giving. It'll give you a bit more of an idea how far to stay underneath.
It'd also be interesting if any ATC guys here would be able to tell us when an aircraft is considered to be infringing, is it when there transponder shows it or do they allow an extra 150ft on the transponder readout for any possible errors.
ie if under a 1500ft airspace would they wait untill the transponder clicked over to 1700ft before the alarm bells ring or is it the moment it goes onto 1600ft.