I think the thing that strikes me is that there doesn't seem to be another alternative for you... You can go "by the law", but you do risk then pissing people off to the point that they begin to look for reasons not to employ you any more. Depends on a few things I guess, starting with if you're in a high demand sector, through to how much of a douche your seniors are (sounds like they're pretty douchey to me).
First thing that I would do is look for an alternative, try find a spot that's out of sight of customers or clients etc, but within easy reach so you don't have to walk a mile to get to it. Try that spot, be careful if people have to walk nearby that you don't blow steam in their faces, and see how you get on - one single complaint and you'll be forced into the shed. If he bitches at you, then you tell him you rightfully don't want to be surrounded by smokers, and that you're willing to put some effort into coming up with an alternative. It sounds like that may be your current situation though... The problem is, if you charge in with a list of laws and regulations, he's gonna come back with a fuckton more about your every day job, and start enforcing that shit (making life hell for you, basically). That's what I'd do if someone approached me with some sheet of rules.
So, next step is to try persist a little. Have a chat with him, or even skip him and jump higher up. If it's a large company with many offices / stores / warehouses / whatever, you may want to find one of the building executives, if it's a small company, you may well be royally screwed, but either way you should persist to try find a tradeoff, where you come off favourably but he also feels like he's won. If he won't accept it, you could go to Citizens advice and see what they suggest, I imagine they have some generic templated letter that you can submit to the employer to get them to do something...
Citizens Advice - Work
Just take my word for it, you really don't want to take the balls deep legal approach. I've tried it before at a previous job, and 2 months later I was forced into voluntary redundancy when the recession hit us for the 5th time and shit hit the fan. I *was* one of the favourites to stay at the company, until I started my crusade, at which point I immediately hit the bottom of the list and was first in line for demotion and dismissal.
