It would have to be a new SIM and new number, on a separate contract.
If you are only using the tablet for mobile broadband, you may be better looking for a mobile broadband provider. BT no longer provide just a mobile broadband only contract.
There are a number of PAYG mobile broadband providers, which may suit your usage better.
Have you tried the SIM in the tablet - works fine for me - I checked with BT & they said is fine also.....
Is BT about to launch data only tariffs? There are a lot of data only packages now appearing in the change plan area of My BT
That’s a very good spot, seems they are upto something.
I would still look at teathering and/or upgrading to plus, over an additional contract. Or as mentioned before maybe something like a giffgaff data bag.