That deal is:
300 minutes, Unltd texts and ONLY 250 MB (That will be done with Cortana info gathering for 10 days only) and for 24 months!
?540 for such low allowance? You can get sim-free Lumia 830 for ?300. So ?10 a month for 250mb of data?
Giffgaff gives you 1GB 4G for that price.
Three gives you unlimited 4G for ?15.