It's a nightmare!! Although it's lovely watching your clothes get baggy, the practicalities of buying new clothes that will only last for a short amount of time is expensive!!
I've bought a pair of jeans off ebay for £1, only Asda ones but they're ok - I think that unless you have money to burn then try and see what you can get on ebay, or try the cheapy ranges at Supermarket, Asda and tesco do cheap jeans, (though I find that the CHEAP ones aren't a very flattering shape) if you can put up with that then that's an option.
As for Jackets.. I'm so nearly too small for my denim jacket and will be selling it soon. It's a size 20 from Dorethy Perkins and it's a lovely flattering shape!! So you can have first refusal.. I should sell it now really because I'm starting to wear 16's now but I have that old 'I might put on weight again' thing .. Stooopid... sorry.. I've gone off on a tangent