It sounds like the cost of fuel is around £400 per hour, which is split between the 2 pilots
When I do the bi-annual dual checks with the owners in a JP3 group, if we fly "wings only" we are pushing to get 35-40mins airborne. That is for takeoff, steep turns, a couple of stalls, a few aeros, a PFL, and say ~3 circuits, all interlinked with ~180K transits. I tend to ask the owners to "half tip" it (another ~700lb / 200L), which means we can comfortably do ~50mins-1hr airborne.
The "wings only" means starting with ~1450lbs and landing with 600lbs = 485L. I'll let you do the rest of the maths...
MoD rate fuel sounds great, if you can get to/from there to uplift it.
Splitting the fuel cost is great, but think carefully how much time you will actually achieve that? By the time you have done your training, and recurrent checks, that is a fair number of hours paying all the fuel.
In practice, given the JP speed, a typical flight need not be an hour. A lot of 20-25mins airborne trips in the logs.
I am not trying to discourage JP flying - it is a different sort of flying well worth experiencing while you can. But it is a real shame the % of people who buy in, and never progress to FHT and beyond for finance / time etc.
NoD