Ring up and do it today. It's £3.50. That's one pint at the pub (well it is down here), 2 bottles of coke, a can of deodorant at a rip off corner shop. It's not going to break the bank & that's a fair increase in speed.
That being said have you shopped around if out of contract. Some of those cashback deals might make it 'cheaper.'
That being said if you use wifi be aware that 40Mbps can easily go through a few walls whereas 80Mbps cannot.
In my household I see it as a little 'traffic management.' Where I am can get 74Mbps and then it slows down to 10Mbps in some parts of the house. Those with the slower speeds are the ones torrenting usually. So the poor wifi signal stops them interfering with the rest of us.