There is no slippery slope because Britain, like Canada, has ALWAYS had tight gun control. These people want to go UPHILL (shredding the metaphor a bit here) and are pissed because the gov't won't give in.
Actually tougher gun control in Canada only came around in the last 30 years or so... I hate to quote wiki but it's pretty thorough on our laws.
Gun politics in Canada - Wikipedia, the free encyclopedia
Point is there's no available tool other than a gun that is as easily operated by an untrained individual, able to inflict the greatest amount of destruction in the shortest period of time and allows a perpetrator physical distance from his(her) victims maximizing his chances at staying alive and thus, bringing more carnage.
How about a molotov cocktail? Go to gas station. Buy bottle of pop and a lighter, take a paper towel from the washroom. Drink pop. Fill bottle with gas, lighter fluid, etc. etc. etc. Stuff rag in, light it, throw it at people...
Maybe the reason these sick fucks choose a gun is it's the easiest thing to turn on themselves after? Most are cowards after all...
I don't think anyone is saying there shouldn't be reasonable regulation (and again I think our current regulation in Canada is reasonable) but outright bans are not the answer.