6. Fair Usage: If your Contract Allowance provides unlimited data access within the UK this will be subject to a fair use limit of 3GB. If we think your use is excessive or that you are abusing the service we may ask you to moderate your usage and/or we reserve the right to charge you for the excessive element of your usage at the standard out of bundle data rate.
So why have I been given the option to upgrade from 3GB data to unlimited if there is a FUP at 3GB?
Just in case you missed it, that's the limitation on unlimited broadband (a data-only SIM for tablets or whatever).
If you are on a pay monthly/pay as you go plan with unlimited data, the FUP threshold is 3½ GB and it doesn't "cut off" or cost extra, it just runs at snail's pace for the remainder of the calendar month. The FUP for that is here.
If you are indeed talking about a data-only plan for your tablet, then it's the most bizarre thing I've seen. At least with the phone SIMs it's not a limit but a threshold where it slows down (semantics - some people might still call it a limit).
But a FUP limit on unlimited data for a mobile broadband SIM.is still a limit. That's a misrepresented product right there.
If you use 4GB per month, that works out at 133 MB per day, so on "unlimited" you'll run out of data sometime on the 26th, which means you'll spend the last few days of the month at 2G speeds but not pay any extra.
If this happens on your current contract, you pay an extra £2 per day (so £8 per month total) but at least you get to do it at Virgin's full speed.
It all depends what you're doing with your data. I just dropped to 1 GB because I don't go over that figure except when testing.