tacpot Yes VAT is payable on imports of aircraft parts. So is duty, unless you have a HM Custom & Excise "End Use" number.
In practice, UK maintenace companies & aircraft parts wholesalers, usually all have End Use numbers, and, of course they can recover the VAT paid on import, if they are VAT registered.
On VAT, to a private individual, who is not VAT registered, there is probably little difference, as you will either pay VAT on import, or VAT to the UK parts wholesaler, if you buy it that way.
If the part is installed onto a UK registered aircraft, by a maintenance company that has an End Use number, then there is no duty payable. The maintenance company does not have to pay duty on import, nor does it have to charge the aircraft owner any duty.