Determining the value of your Pokemon cards in the UK involves considering several factors, including the card's rarity, condition, and demand. Websites like PriceCharting and eBay (sold listings) can provide valuable insights into current market prices. Professional grading services like PSA and Beckett can also assess the condition of your cards and assign them a grade, which significantly impacts their value. Remember to research thoroughly and compare prices from multiple sources before buying or selling.