Your dongle may not work - don't assume anything just because you have a UK 3 dongle. They are sister companies but not the same company.
However you can get a dongle unlocked just like a mobile phone so either go along to a phone shop in the UK and get it done before you go, OR do it yourself with the software from here:-
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If you can't link direct from here, right click on the URL and click Properties and then copy and paste the address from the pop-up.
Once unlocked, you can use any sim in it and you will find that the branded welcome screen on your laptop will change to the generic Huawie graphics.
I am currently alternately using an O2 and a Vodaphone data sim in my unlocked 3 dongle.