Out of curiosity, what's the plan with "full unlimited 4G connection" that's cheaper than a line rental?
So-called "mobile broadband" is much more expensive. I'm guessing you might be on a phone contract where the Ts&Cs say you should only use it for the phone and not do tethering etc. But I suppose a VPN to stop them snooping on content and you might get away with it. Was going to say you should avoid iPlayer/Netflix too but given what teenagers are like that probably is normal 4G usage these days.
Edit: I'll answer my own question. Smarty currently offering £18.75/month. And allows tethering!!! I'm gobsmacked.