A metal detector can be purchased from a wide variety of retailers and individual sellers in the United Kingdom. For instance, Tesco direct offers several varieties of metal detectors ranging from ?21.18 to ?85.00.
Another option would be to seek out a group of metal detecting enthusiasts who will help find the right product for your needs and may also be selling used detectors. Gary's Detecting, a UK-based site for metal detecting enthusiasts, offers a page for users to post metal detectors they are selling and those that they would like to purchase. On the same page, Gary also offers a variety of detailed reviews on different detectors and links for where to purchase them. Sites like these, from enthusiasts, will generally offer more expensive detectors as they are more specialized than a starter detector like those available at Tesco or other big-box retailers.