Tango Oscar
Much better to pay the duty, and claim the drawback, so you get the money yourself rather than the group/club.
This is a VERY BAD thing to do; at best it betrays the trust the plane owners/operators have in you (because you are returning the plane with some fuel on which the reclaim cannot be done again until sufficient UK-purchased fuel has been put in to displace all the fuel with which you came back), and at worst they will in honest ignorance do the reclaim again and risk getting turned over by HM Customs.
I had two people (both instructors) do this to me before I discovered its popularity
Fortunately nothing happened.
Any owner/operator with half a brain will have a rule prohibiting renters from doing their own drawback claims. He should of course pass on the saving - few if any schools do this because it's handy extra profit....
The exception to the above might be if you are doing a long enough trip abroad to use up the whole tank contents out there, and the owner agrees.
Also the current drawback rate is 0.281 which gives you a better return than simply not being charged VAT.