If I understand you right, you are only considering metered package from BT (25 GB I think)? Are you not considering other suppliers?
But the point is, it is not possible to make an informed decision as whether one needs an unlimited deal or can get away with a limited deal if they find one that is cheaper without the information on current data usage. With one desktop, two laptops, two tablets, two phones, several internet radios etc to monitor that is not easy to do locally. Even more so when you also have a Youview box which uses the internet a lot, but that usage is not counted towards the cap.
By comparison I am currently considering moving from an unlimited 3G mobile data package to a 4G deal limited to 3GB per month. This I can do as I have been able to see my monthly usage over the past couple of years swing between 500MB to a max of around 1.3GB (bearing in mind that faster 4G speed may cause adaptive video streaming to use a higher quality and that I often make use of free wi-fi hotspots instead when out on the road).
BT Infinity 1 (unlimited)