Too true,
the snag is that if you force the issue you can always find a way round, but worst case is that HMRC will assess what you have for value, then add something for shipment, then add 20% VAT to the lot. If you have a complete jeep which is (now) valued at £15k, the calculation would run something like;
£15k vehicle, plus £2k assumed freight and insurance cost, times 1.1 for 10% import duty, and then the total times 1.2 for the VAT, so regardless of how inaccurate that detail is, there is a bill of about £5500 between that theoretical case and getting a registration number.
Pleading that it was imported in bits, valued at £500 total, will get you nowhere with HMRC, and rightly so, even if that was the case.