Not that likely in the UK to be honest. Yes, there was John Charles DeMenezes and Mark Duggan, and SO19 have a fifteen minute response time anywhere in the UK, but overall, the UK police are significantly less trigger happy than their US contemporaries. You'll get tasered to within an inch of your life, but probably not shot. You will, however, likely do a five year stretch for them. The law here states that if you're carrying anything which can be mistaken as a firearm on the streets, you're treated exactly as if it was a real firearm by the law. It didn't used to be so, but the law was changed in 2003 after a chap held up a series of banks with a tire iron in a plastic carrier bag. He thought that because it wasn't a real gun, he wouldn't be charged with armed robbery - he didn't realise they'd change the law just for him.Though you have to be careful with plastic guns, you might get shot.
We're super-strict about guns over here. I'd never leave home carrying a toy weapon. There is literally no reason good enough for the risk.