It would be about 25% - it's VAT at 20% & the import duty (which is around 3-5%). This is added to the combined cost of the item & the shipping cost.
The other things to bear in mind, especially with electrical items, is that you'll need some kind of transformer to use it in this country (unless it has user switchable voltages) & also if it's made for the US market then the warranty will probably not be valid in this country, and even if it is shipping the amp back to the US for any repairs will be pretty expensive.
If it was me buying this & I was only saving £50 I'd just buy one over here & save myself all the potential hassle.