do any uk company offer unlimited data plan on mobile ?

The supplier "3".

sky mobile were offering some pretty good deal the other day not sure its unlimited though

Nearly all of them offer unlimited but throttle your data when you use too much (think of this like using free wifi in a pub where you can check your Facebook but not watch video's)

GiffGaff have an unlimited data plan for £20 per month. Includes free calls to other giffgaff users, unlimited calls and texts to other landlines & mobiles.

You have to buy the £20 "goody bag" from their website, not simply have the credit on your phone or your credit will be eaten up fast.

Data is always on but there are speed restrictions after 6GB use.