If I were to buy a boat in another EEC country for example Holland, do I need to make any declaration to UK customs or can I simply bring the boat to a UK marina and use it as if I had purchased it in the UK?
The definitive guide to bringing boats into UK is the Customs Guide for Boats Arriving in UK . But yes, I think you can just bring it in and sail . . . subject to checking the detail on the site! Particularly, making sure VAT has been paid . . .
You are right to concern yourself with the pedigree and paper trail coming with any boat purchased in the EU and brought back here. Apart from considerations of unencumbered ownership, which is essential of course, it's valuable to have receipts from first ownerhip onwards. In reality, there may be gaps in this trail, but proof the first owner paid VAT, or the boat is a pre-VAT vintage is essential for the day you come to sell it.
Technically, owners can be challenged at any time in European waters to prove VAT status. Although I've had my papers combed several times over the last 10 years, customs has never gone into VAT status. Others may have similar experiences. UK registration is most important when abroad: be sure you have it to hand!