This page lists suppliers known to and used by Lead Safe World UK. This does not imply any endorsement or recommendation.
Look for product 225-2404 (8 pack) or 225-2404A (16 pack).
It can be cheaper to buy from the USA on Amazon or eBay even including shipping.
Paint and Dust Analysis
You can try your GP, but they may not know about lead toxicity and not want to send you for a test.
Private blood tests. Add the cost of a phlebotomist to their prices. About £25 extra.