It's entirely a union issue I don't think, although they do have power over the way companies act. You need to do what Satsuma suggested and check out the HSE's website. Your workspace has to comply with Health & Safety at Work Act 1974 and various other Acts of Parliament similar to that. It's up to individual companies to decide how best to proceed but your risk assessments should ensure that you're working in a legally acceptable space. It's your employer's repsonsibility to make sure you're warm and dry, have clean water, a place to eat, toilets and PAT tested electronics - stuff like that. You've got them by the balls if you haven't got proper toilet facilities!
HSE's site is ok; it's fairly public friendly. You can contact the HSE by regional office and you can complain about your employer anonymously.
http://www.hse.gov.uk/contact/maps/index.htm