There’s no loophole there. Unless you have paid and not reclaimed the VAT you are basically cheating the taxman of his VAT. Lot’s have done it but that doesn’t make it legal. It might depend on whether you have glazed second row doors, but since Grenadier don’t qualify for VAT I suspect that neither does yours after conversion.
The commercial vehicle with aftermarket seats is not a loop hole- its, well, let's just say, at the risk of offending, it depends on the "flexibility" of your accountant's approach...
VAT men are rarely flexible if caught though, and that's before the insurance ramifications of a modified vehicle. Plenty of them about mind, and risk is a personal choice I suppose...