I've never seen a dollar price converted to sterling when buying from the USA. (Simply put the dollar price into Google, followed by 'in pounds' for a conversion. The rate used by your card issuer might vary, and there could be a small fee to pay as well, but it will give you a reasonably good guide to what will be taken from your card).
Not all US traders on Amazon will ship to the UK, so check that they do (and that the shipping costs aren't extortionate).
For electrical items, remember that they'll be designed to work on 110 Volts , not 230 Volts. There may be compatibility issues with other items. (US DVDs won't play in the UK unless, unusually, you've got a region-free player. Mobile phones might only work on US frequencies and/or come with apps preloaded that only work with US services).
Although you'd have the backing of Amazon if the item you received was faulty (or simply did't arrive), remember that you've not got the backing of UK/EU trading standards, that you'd have with domestic purchases, if you find a fault later. Some manufacturers won't honour guaranties on what they view as 'grey imports'.
Check on what the import charges might be (or get us to do it for you). Unless it's a fairly low value item, you could get hit for Import Duty, VAT and a customs examination fee.
Import Duty varies widely, depending upon the type of goods being imported. With imports from the USA there are also EU sanctions to take into account. For example, Import Duty on cotton-based clothing is normally charged at 12% from non-EU countries. However the EU increased that to 27% for imports from the USA because they're in breach of World Trade Organisation rules through providing state subsidies to their cotton farmers. Import Duty on several types of clothing, and on footwear, was then upped to 50% as a reprisal for the USA imposing trade tariffs on steel imports from the EU.
VAT is charged (at 20%) on the cost of the goods AND on the carriage costs AND on the Import Duty. (Yes, that really is a tax on a tax!)
On top of all that lot there's an £8 customs examination fee to pay.
[Notes: There's no duty to pay on anything valued at less than £135. There's no VAT to pay on anything (except booze, baccy & fragrances) under £15. There's no customs examination fee to pay when you don't have to pay either Import Duty or VAT].
So you'll avoid any charges if you're buying something that only costs a tenner. However if, say, you're buying clothing (of the type that's hit by the 50% tariff) priced at £200 (with £20 carriage), you'll pay £100 Import Duty, £64 VAT and an £8 inspection fee, meaning that you'll have to hand over £172 before you can receive your purchase.