As a Brit, I can feel safe in the knowledge that noone has invaded our country for just under 100 years. There are three reasons for this:
1. There is a magical object called the English Channel which acts as a wonderful way to stop people just walking into the country.
2. Who seriously wants to have Britain? The weather is awful, which would cause all the invaders to get depressed and go home.
3. Finally - three words STEALTHY BRITISH NINJAS. You may be thinking "what ninjas?" well, if you could see them it would be kind of counter-productive.
Anywho... on topic response is dependant as many have said before on the invading force. If Britain was ever to become a dictorship *cough*Tories*cough* then any democracy can come on in. However, if your just a nutter with an army who wants to have some land, try somewhere else, like, I don't know.... Antartica?
If pressed for an answer, I would say: "I will fight for my Queen and my country. For the tea and the pinstriped suited buisnessmen I would fight for *INSERT STEREOTYPE*"
Just kidding. The royal family would be the first to leave the country in an invasion so fighting for them is not worth it. However, no-one desecrates my tea. NO-ONE.
TL;DR - The ninjas are everywhere, yes I would fight but only if someonetries to take my cup of tea....
[EDIT] BTW, AndyFromMonday, I have had more experience in these matters than you might think. THat's all I'm willing to say on the issue.