Nothing offends me like the smell of dog. I’d actually rather have the smell of vomit wafting around me.
Wow. 
I find it completely hilarious that the dog owners on here think their dogs don’t have fleas. Of course people are the main spreaders of fleas, of course. And of course a fleas natural environment is a carpet. It’s funny they’ve never lived in my carpets though. Could this be becasue no dogs have been in my house?
Probably not! I'm going to assume you don't have a cat. It's cat fleas that are the problem, no dog fleas, despite the "mangy, flea-ridden dog" image you are trying to project.
OP, Hobbycraft stores on retail parks are VAST. It's not really a "confined space" in the sense that a lift or small cafe might be. You may be unlucky enough to encounter a dog in one now they're allowed, but I think it vastly unlikely all the crafters who have been previously leaving their dogs at home when they shop for their hobby will be taking Fido to buy fat squares and hot glue guns now it is "allowed". And if you did spot one, you could get away from it in that big space very easily.
You asked if you were being unreasonable and I am afraid I still think you are - nothing you have said has convinced me otherwise.
It sucks that you are allergic, it does. But I still don't think a retail park superstore has much to worry you - just because dogs are now 'welcome' does not mean that Hobbycraft will be over-run. It's just unlikely.
There are also some mildly hysterical people on this thread with the disgusting-dog-germs stuff.