Basically you call them and ask for a MAC code, which they have to give you free since 14th of this month.
However about FUPs.
Most ISP's have these, even if they don't advertise the fact. They can be found in the very small print somewhere on their sites.
I'm with Virgin and there's is 40Gb (enough for me) but still advertised as 'unlimited'.
And before you switch make sure you are going to get a better service.
'Out of the frying pan into the fire' is worth pondering on.
If you are getting a good reliable speed then that's important criteria, as a lot of ISPs are grossly overloaded and peeps are getting dial-up speeds on BB at peak times.
Check out UKFSN, they may well have a FUP as well.
Comments here seem very approving of UKFSN,
http://www.dslzoneuk.net/isp_ratings_comments.php?isp=100
Problem is when everyone goes over to them, are they going to cope, believe this happened to Vivicity.
Good luck,
Mike.