gawd knows. There are some funny rules.
The way I understand it, you cannot "Rent" an N reg in the UK in the traditional sense, you can however "dry lease" one. Same thing really, you essentially rent the aircraft dry and pay for the fuel yourself. Not sure if you have to say beforehand "I'm going to lease this aircraft for 5 hours" or just take it away and bring it back when done.
Probably something to do with the DfT......