As a Jersey resident and frequent eBay seller, can I just clarify that it's not eBay that add 20% but HMRC who go by the information supplied on the CN22 customs declaration, ultimately the sender.
For any future purchases, as long as they weren't a trader or you had an understanding seller then you could ask them to down-mark the value and/or declare the goods as a lower value than they are since it would otherwise make it uneconomical to purchase, this would be in their interests to do and wouldn't cost them anything, although strictly not the correct thing to do.
The VAT threshold is £18 for imports to the mainland, so anything under that will sail through, anything declared over will be liable for VAT.
we have exactly the same issue here, we have our own version of VAT called GST, if I buy an item for £239 it clears customs, a pound more and customs will seize it, send you a letter in the post and ask for the duty, a process that can take nearly a week. I had an issue last year where somebody sent me a motorcycle part that cost me £35 but for some unknown reason they wrote £275 on the declaration (I can only assume they went for a new-for-old replacement value) needless to say that I had the choice of paying the duty or losing the (rare) item.
just out of interest, was it a private sale you bought from or a trader? I ask because I need a shell and didn't think anybody here sold them!