The thing with sizes is, every single shop seems to measure differently! A size 18 in Evans for example could easily be a 22-24 in Sainsbury's. In 2009 I lost 4st, going from 16st10 to 12st10. I started wearing size 20 jeans, 24 tops, and two size 32 dresses! Went down gradually, at some point I was wearing a single size 12 top I found in Sainsbury's and size 16 jeans, but the rest of my tops were 18 so what size was I? No idea!
The best thing I did, and I am doing again, is pick a brand and stick with it. Last time I did Matalan, jeans were £12 and I already had size 20 so I bought them in an 18 when I lost a bit, then in 16 a bit later. I even had 14 for the next step but life got in the way and I stopped the diet. This time it's M+S, £12 a pair jeans. Currently in 18, and in my room I have a pair of 16 in the same range. When I fit them, I'll buy 14. I know that I have most definetly gone down a size when they're from the same shop! If that makes sense.
It's one of my biggest motivations. I'm one stone down, 6 more to go! Every time I go in my room I see the jeans I want to wear, on a size 16 hanger!