I agree on boats it can be a bit of a sod and they really have a thing about them.
But for a light aircraft if you wait until just before an engine change strip out the avionics and send the engine in. The hull value is relatively little on a light aircraft minus engine and newish avionics which you have receipt for so you could do it at market value.
Then get it back and do the engine change put the avionics back. Then you have the aircraft back with a VAT receipt for the hull and one for the engine.
For a 15k Cessna I reckon you could get it sorted for 750-1k VAT bill on the hull. Maybe less if it hasn't had the SID's done on it yet.