You need to go to a chemist and ask for Ketostix lisa. They're used mainly by diabetics (I'm diabetic and have been using them for years). However, I'd advise you NOT to get them from Boots but to go to a small independant pharmacy as the assistants at Boots can get a bit huffy about selling them if they know they're going to be used by people on a VLCD (no idea why: they're not a drug or anything!)
The last time I bought them from Boots, I practically had a toe-to-toe row with the assistant - but as I AM diabetic and fronted it out, she didn't have a leg to stand on. Now I wouldn't buy them there on principle!
They cost about £5 for a tub but they last ages & ages. Hope this helps
