A C88 is the common EU import proof and looks like this :
http://www.hmrc.gov.uk/forms/c88%281-8%29.pdf
The old, printed sort were green and white paper but the newer ones are computer generated and just black and white.
If you have one your aircraft is imported. That doesn't mean that the importer didn't claim the VAT back! But it is usually enough to send the Douaniers on their way because they don't seem to understand the subtleties of international VAT.
Import into the EU is only done once - so if yours came in through Denmark it would not need to be "imported" into the UK.
After import the aircraft may be sold "plus VAT" or "VAT paid" and the status of the current owner determines which it is. A business owner, who has reclaimed the VAT paid at import, will sell plus VAT. A private owner cannot reclaim VAT and so sells on as VAT paid.
You will need a proper invoice from the seller showing VAT paid status. If you have the C88 keep it as a backup.
Anything that was already in the EU at the time the single market was born is "deemed VAT paid" and can be tricky because there is nothing other than your purchase invoice to back you up.
Moral of this is to keep ALL the paperwork to pass on when you sell the aircraft.