It has been reported that a security guard at the stadium found the first suicide vest, it seems the bomber then tried to get away from security and all three detonated their bombs, presumably because they realised they weren't going to get inside.
Would we be so lucky here? Considering our security industry is mostly on low wages and long hours? Considering the poor vetting of many staff? I have worked at various bits of our "Critical National Infrastructure" and at many events. In many cases the procedures are dire and full of loop holes. Many of the staff I have worked with work on the basis of "It'll never happen here", not just a terrorist attack but anything. The sussed out ones are in the minority.
I've turned up to take over from the night shift, only to find he has gone on his patrol yet left the front door of the building open for the cleaners to get in, and he's patrolling whilst on the phone to his wife. Or on other occasions he's gone to the loo and left the keys on the desk. At another place I jumped up and down in front of the single camera in hi-vis without the person noticing because his laptop was in front of the monitor whilst he watched a video.
As for the procedures they seem more about making a show of security than actual security. Places that search vehicles but not when it's raining, or search vehicles except the company's own vans that go in and out all the time. Searches on people that don't search anywhere "sensitive", pretty much not searching the area that would be covered by a T-shirt and shorts. Events where I've turned up and said I work for the security company and I've been let in without any checks and could have gone anywhere, then no-one checking my licence actually matches my face, just write your name and number on the time sheet. Then finding colleagues that don't speak English so obviously weren't the person that sat the exam.
Basically the standard of security at places where it really matters is dire and it wouldn't take much to compromise it. It will only be a matter of time before a Paris/ Mumbai type attack takes place here. Their targets and tactics may be different, they might hit festivals or power stations. It probably isn't imminent, but they will want another big attack in Britain.
As for closing the borders, we have nearly 20,000 miles of coastline. How on earth is anyone going to keep that secure? The Mumbai attackers did not come through passport control, they came on a freight boat. It would be incredibly easy to land a small boat on a beach and get men and arms into the country from there.