If you’re declaring the income from the driving you can claim the costs incurred to achieve eg R&M etc. I’m no tax expert but I’d suggest you keep an honest vehicle log book and all your receipts.
% of vehicle use against income will be based on a three month sample (at least that was my experience) and that % of income earning vs private use can then be generally applied..... (ie choose your three month sample wisely).
Frankly I ran a log book 100% for years anyway but it was never queried
Note that costs to earn will include your ph etc too so it should compliment your existing interests on the island anyway..... (if I understand correctly what you do, referred earlier somewhere previously, I think)
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