Hello - OK - hover over the address where the email has come from - if it says a person's name, it's a SCAM.
Don't click on any links as a matter of security anyway …. but rather use a different browser (if you have one) and log into your account. You can check how much data you have used via your account.
I have never had an email from BT saying just "hello" - it's usually by name so I would be very cautious indeed .... and if in doubt, you could always call BT to double check.
Just realised, you've mentioned BT Infinity ...... you have unlimited usage 🙂 However, if you have Infinity FIBRE broadband, then the limit is 30MB
Hope that helps
If you access your router stats you can see each device on the network and how much bandwidth they have individually consumed ( count both the upload and download )...it also shows the up time , so if the device (router) has been 'up' since it was installed, it will show the total usage, if it's less than 21Gb, then the email , ignoring its origin, cannot be true, but if the router stats confirm what the email said, then you are probably not on a suitable package for your usage...enter 'hub.home' into your browser and check the 'status' and 'my devices' tabs