You need to realise just how ignorant of guns people are in the UK, there's no real way to put it in to words. You would without a doubt have the armed response bearing down in you in minutes regardless of contacting the police in advance or anything like that.
Honestly, from the experiences I've had working around even cadets and reservists (who actually do have a little bit of training and experience, unlike joe civvie who has none), you might well encounter problems if you turned up with a bright yellow super soaker while also wearing airsoft kit. Many people are literally that fckn retarded when it comes to these things, I'm really not exaggerating.