Which network? Three has an offer like that but I found connectivity quite weak once you leave larger cities. Although that appears to be the issue with most/all networks here. I currently use vodafone and can't remember when I last had 4G outside of major cities.

I think all the networks can be hit and miss depending on the specific location.

Apparently it can also vary at any given moment, depending on equipment failures and access to equipment issues.

I'd recommend getting a couple of cheap-ish pre-loaded data SIMs from EE and Three, and have your main data SIM with someone else.

I carry a 500MB/month EE 4G SIM that was cheap in Argos, in an unlocked 4G EE wifi router dongle. I have an O2 (MVNO) SIM in my phone.

Plus an assortment of pre-loaded (but not activated) SIMs in the event I need Three, or need more data.

I've not had any luck at all with Voda, but should I give them another shot?

It was a friend who got it last month, but I can't find it now. I thought it was EE, but I only see 30G offers on their website now. If I find it I'll update.