It's all digits, I think there is a basic data link at whateever speed can be attained over which data, SMS, phone calls are carried.
But giffgaff are on O2 and O2's coverage can be a bit poor. They are only 2G here, all the others 3G and EE 4G.
The big gotcha with many tarrifs is "no tethering" ie the data use is for the device with the SIM only. The networks can tell to a limited extent as well. TPO don't spot my tablet via the phones WiFi but if I connect the home LAN they do in a few tens of minutes.
It might be worth a look at Asda Mobile, EE (but 3G only), allow tethering, have data only plans. Still worth doing the maths on p/MB though if you know how much data you are likely to need. Their cheapest data is a normal bundle, 6 GB Data, Unlimited Mins and Texts for £20 - 0.3255 p/MB, the biggest data only is 4 GB for £15.00 -
0.3662 p/MB.