No, you are not "in the UK" virtually speaking. Somehow you navigated across the broad Atlantic (or Pacific, or 49th Parallel, or Rio Grande, or whatever) to ebay.com (the primarily US site). Here most of the listings will be denominated in USD (US Dollars, displayed by ebay.com as "US $"), and those that aren't will have an "approximate"* conversion to USD under the actual price/bid in the currency of the listing or, on some pages, INSTEAD OF the actual price (this is the default setting on search results). Be aware that any bid/offer you make must be in the currency of the listing and when you pay the seller must be paid the full price in the currency of the listing (eBay's payment processing contractor will convert your currency to the currency of the listing, but see below).
If you go back across the big water to ebay.co.uk (or similar for your home country's eBay site, e.g. ebay.com.au, ebay.ca, etc.) you will likely find something similar but reversed: a higher percentage of listings in GBP (Great Britain Pounds, displayed by ebay.co.uk as "£") and some approximate* conversion display of those that aren't to GBP. Keep an eye on your address bar, especially when you are asked to login, so you don't accidentally emigrate here again.
*Be aware the "approximate" in this case means ALWAYS too low: they are calculated using readily accessible "mid-market rate" quotes (an average of wholesale buy and sell rates of several large banks) while someone with funds in the approximately-converted-to currency (USD on ebay.com, GBP on ebay.co.uk) will have to pay someone their retail "buy rate" to pay the seller the correct price in the currency of listing; figure about 4% higher if you let eBay checkout do the converting.
