With the weight of that boat and a 200 4 stroke, i wouldn't have thought you would get under 1L a mile. We have a 6.3 Humber OP with opti 2 stroke, not much less economical than a 4 stroke. At 25 mph we get 1L per mile.
4 hours at 35mph. You will have travelled 140miles. at £1.10p/l you have spent £154. Give or take some economy and stopping starting, even if you reduce the distance by a third due to pottering around and reduce it to 0.75L per mile, you will have spent £77.
Lovely looking boat though, if not a tad underpowered perhaps. Though a nice cruiser.