Bought an Alpinestars Al Mega from the states. Cost me a bloody fortune to get it over here in one box (£200 with DHL) and I had to take a risk and declare it's value as $100 so I didn't get stung with customs charges.
Customs charged me about £15 if I remember rightly.
FWIW and from what I've read since, your much better off having the bike sent in 2 boxes (wheels in one, rest in the other) - that way, you'll save money on the postage because each box will be smaller (courier companies base a lot of their charges around the size of the box, rather than the weight).
It's up to you whether you want to risk having the bike sent with a low declared value (lower customs charges but less cover), or have it sent fully insured with the proper value on it (higher customs charges but more cover).
Personally, I'd take the risk if the bike only cost a couple of hundred £. If it was a much more expensive bit of kit, then I'd swallow the import duty because I'd rather it was covered in case anything went wrong!