Users in other countries are being redirected to the US site when selecting categories to search on the main eBay page or search results page without using any keywords. Avoid searching that way; you can add keywords and search, or else choose the categories using the "Shop by category" links on the left side of the search entry field.
Try erasing your ebay.com browser cookies -- that way if you are redirected again, you will be forced to sign in, making more likely you will notice.
Sign back in to the UK site here:
https://www.ebay.co.uk/
If you are actually trying to place a bid on the US site, or problems persist on the UK site, what browser are you using? Have you tried another?
What page are you starting from -- item page or bid history page? Try the other page to see if it makes a difference. Or try using the "Place bid" button without entering a number to see if you can proceed from there.
Are you bidding greater or equal to the minimum amount?
Are you seeing any error message? What is preventing you from proceeding?
