Prices for fishing tackle bought from the US can seem very attractive compared to what you'll pay for the same item in your local shop. but it isn't as simple as all that. There's a threshold value below which you don't pay import duty, but it varies depending on the nature of the product. I've bought Gudebrod whipping silks from an Americal internet retailer and paid only shipping costs.
Once you exceed that threshold, the percentage appropriate to the type of item is payable at point of entry into the UK. If you were to collect from HM Customs & Excise you'd pay the import duty and walk away with your item; in practice most people use a parcel carrier door-to-door, so once it arrives in the UK Royal Mail take over and clear the item through customs on your behalf, but before they deliver they'll charge you the import duty plus a handling fee.
There's a scale of percentages and other advice on this site:
http://customs.hmrc.gov.uk/channelsP...CE_PROD_008654
This link indicates the relevant category and commodity code applicable to fishing tackle:
http://www.businesslink.gov.uk/bdotg...false&x=21&y=7
Add up import duty, shipping and handling fees and, as 3lbgrayling suggests, you'll pay in the region of 30% on top of the purchase price - but it can be less, or more. The safest option is to work out exactly how much before you buy, as it isn't unknown to end up paying more than if you'd bought from the UK.