The TK Maxx near me had a good range of Craghopper, Sprayway, Regatta etc coats and trousers but I couldn't find any waterproof overtrousers. As well as the suggestions above try shops that sell fishing gear and also those that sell work clothing, like Greenhams or Makros etc, they should have waterproofs there.
------------- ---Phil---
"I distrust camels, and anyone else who can go a week without a drink."
We buy ours at the farmers place (think they were £8 a pair this year). You could try horse shops or things like that.
They are cheaper than the likes of Millets, outdoor shops etc cos they aren't 'fashion' aimed.
We always buy our wellies there too - £5 a pair for the green ones. They have the all in one suits too.
Where do you live? Where are you going camping? Maybe you could get some when you get there rather than taking with (just remember to leave a little space)
Depends if you want something that will keep you totally dry , some cheap ones are not even shower proff also condensation builds up, get a gore-tex pair bit pricey but will last for years