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Can anyone tell me if it is a good idea to email my resume before I leave to the U.K.? If so how much time in advance should I do it?

It's up to you - I'd prefer to have a job lined up before leaving, but it's up to you how you want to do it. It can go either way.

What if an interview is requested from the employer?

Then simply tell them the situation.

It's really not a big deal. Tell them you're happy to do an interview by phone (I've done that a few times) and tell them when you're planning on being in the UK.

Its up to you if you want the job badly enough, then you'll go for an interview. It doesn't hurt to apply though. Just be honest with the employer about what your plans are.

It really can't hurt to send out a resume, called a CV here in the UK. I didn't do it when I moved here but when I moved from Canada to the US I did. I made a note on the resume saying when I would be available for interviews. I assume you will be able to work here legally, in which case you should note that on your CV.
Personally I don't think there is a whole lot of point in sending it out more than a month in advance, though I guess it also depends on what you do. Then follow up when you arrive.
Good luck.

I agree with the 1 month 'rule' - maybe two. I have noticed that hiring often occurs in the spring and early autumn. Summer and around Christmas is so slow, the molasses running in January would be faster! Make sure you are allowed to work in the UK - if you go through a UK agency on the net, they will have you indicate it in any case.