As a single person, I am currently averaging 130GB a month. It varies quite a bit though depending on how much time I have to watch streaming video (NetFlix, iPlayer, AmazonVideo).
The more people in your household who have devices capable of streaming video, the more data you will use. If you have 3 children and they each have a device on which they can watch NetFlix they could all stream something at the same time, if enough data-speed.
I have a Sync Speed of 77.4Mbps. BroadbankSpeedChecker says 30.8Mbps Download. SpeedTest.net says 22.18Mbps Download (though had peeks of 55Mbps). So Sync Speed (the speed at which your router connects to the network) and the speed that online tests record are different. Thus go for as high a SyncSpeed as is offered. It is typically an up-to 38Mbps service (used to be 40Mbps) or an up-to 76Mbps (used to be 80Mbps) service. Think lower numbers are used now so that it is more realistic of what SyncSpeed you could get, as no one got the higher figure.
To shop around for packages you need to know what Market your exchange is on. Use SamKnows to locate your Exchange, then look towards the bottom of the detail to see Ofcom Classification.
Market 3 means there is lots of competition, so you will get good pricing.
Market 1 means that only BT Wholesale provides Broadband, so choice will be limited and prices higher.
Plusnet provides a helpful guide to Exchange Markets - Plusnet: Low Cost Areas and shows how their costs vary between the different exchange markets.
For example Unlimited Fibre, Broadband Only, is £17.49 in Market 3 but is £24.99 in Market 1. (prices are before any special introductory offers)
A lot of providers now are offering Unlimited data on Fibre products, so data usage may not be your deciding factor, it may be more than due to location your maximum sync speed may be limited to 38Mbps, so no point paying for a higher service.
You will get a lower cost for Fibre Broadband by taking out a package that includes Phone Calls. This means that your phone gets billed from the broadband provider. There is a Monthly line rental fee.
Some provides will let you pay line rental a year in advance (useful if you are not intending to move house anytime soon). This may be called Line Saver or something similar. It does not include any calls, so depending how much you use your phone it may or may not result in a saving.
Personally if I was currently looking for Fibre Broadband, I would go with a provider who provided Unlimited data. With BTInternet I think only Infinity2 is unlimited. With PlusNet (part of BT Group), they have two Fibre packages which are both Unlimited data. Contract lengths vary, 18 months is typical.