White-Tie Affair Security (UK)

For my sci-fi/romance novel, I'm planning a white-tie ball that's to commemorate the anniversary of First Contact. It takes place just slightly in the future, within an old castle on a cliff, an hour or two from London. The Prime Minister will be in attendance, along with dozens of alien ambassadors. Very posh, and since the world in general doesn't know about the existence of aliens, it's all very hush-hush and security is top notch.

What sort of security precautions would be taken? Armed guards, I assume, at all entrances. Would they check your invitation and ID upon leaving the building, or just upon entering? Would there be metal detectors at all entrances? Would the invitation be a card, or something electronic that can't be duplicated? Would there be retina or thumbprint scans?

Basically, I have a character who needs to run out of the castle, and I need to know what happens when she reaches the side door. Will she be stopped and forced to show her ID, just to leave? Will they permit her to go without question, or since she's obviously running, would they ask what's going on? Or is that none of their business? When the guy chasing her shows up, will they stop and question him?

Thanks in advance!

So, would she have a problem leaving the castle? She works for the government, if that helps.

Okay, so I'll have her stop and compose herself before she gets to the guards. And I'll mention that there is a security cordon around the castle, just to beef up the security a little.

Um, what's a secret squirrel?

Ah, okay. It's not really relevant to my story, so I'll probably leave it alone. Most of the scene takes place outside the castle, after she's fled.