While I'm not from the UK, I'm pretty sure this is something that we all have to deal with no matter where we live. Something that is equally annoying to not changing sizes as quickly as one might hope for, is that depending on which store you are shopping at, you may wind up having to buy a totally different size all together!
My advice to you is to give it a few weeks more. Loose jeans turns into needing to buy a belt or a new pair of jeans