noogles couple of points. Just out of interest, why did you return as a sales advisor when you'd previously been an assistant manager? Did you request a reduction in your hours on your existing job and was it refused?
Secondly, legally you don't actually have to have a 20 minute break until you've been working for 6 hours, not 5. Obviously I wouldn't normally encourage working with less breaks, but just to let you know that. I am assuming that the 5 hours with a break rule is an internal one at your company - everyone else has a break every 5 hours so they couldn't make an exception for you?
If your employer want to change your terms and conditions they can't just do it, they must consult with you, listen to any suggestions you might have or concerns, and you can refuse. Having said that, if there's genuinely a business need to do it, it might be tricky for you to refuse to consent, they could force it through and if you still refuse you'd have to claim unfair dismissal, a tricky process that you'd be fairly unlikely to win if there was a genuine business need.
BUt they must go through the various hoops, so make sure they do, see here about what they should do, and here about what happens if you refuse to accept the changes.
As you can see, they must consult with you or your union if you have one, at which point they must listen to suggestions. Can you really not do the extra they are requesting, or would you just rather not? And if you can't, can you think of any other way to solve their problems, any other solutions to the business issues they are referring to? Talk to your union if you have one about this.