Hi Greeneye
I work in the Civil Service and we're allowed a break in the morning and we can also take a break in the afternoon if we want (although most people don't bother). I think this is normal in most civil service departments so it sounds as if some local manager is just taking it on themselves to change the rules. The fact that you're not even allowed have a cup of tea at your desk sounds really petty. I would go back to your union rep and insist s/he does something about it.
Also, ignore some of the stupid comments on here. Any mention of the Civil Service will very quickly bring out some private sector people puffing their chest and talking about how hard they work, how they never stop for lunch, don't get home until midnight and have to be back in at 3am etc etc. It gets like a Monty Python sketch sometimes.