Depends what it is.....Is it an older AST300 type or a more advanced PCATD type of thing like an Elite ATD (instruments on flat screen)?
For FAA IPC, I would pay 50 GBP per hour plus the instructor (FAA CFII or IGI). I may pay for sim alone if I wanted to practice instrument stuff (say with a mate acting as ATC), but wouldn't pay for instruction if it wasn't loggable. I also wouldn't pay the "FAA Approved" rate as it is not loggable without a CFII (or IGI) and the approaches don't "count" for anything....so say 30 GBP per hour.
Have you thought about renting it out to FAA instructors for a reduced rate? Say 35 GBP per hour for blocks of 10 hours or more? They can then bring in their students, charge them 100 GBP per hour all in, yet make 65 GBP per hour, and you get your sim used and paid for.....
Cheers