Definitely 3G/4G mobile data. I use a Huawei
MiFi with an external antenna on the roof, but other brands are available. Much better to use a
mifi than just to tether your phone, for reasons too complex to explain here.
As for the network to use, I would suggest EE is by far the best, both for coverage and for data speeds.
Having said that, I use Three, simply on the basis of cost.
At home, I use Three mobile broadband, which costs £20 per month for unimited data, texts and calls. Don't use the texts or calls much, but there you are. It is easily good enough and allowed me to cancel the FTTC broadband and the phone line it needed. Three mobile is much faster than the FTTC was.
When away in the motorhome, I put a prepaid Three sim into the house's router and take the unlimited sim into the motorhome.
If you use a lot, Three is much better value, but if no more than 20GB per month will do, I suggest getting EE because it is so much better.
GiffGaff and O2 are utter crap in my experience.
Yes, GiffGaff is cheap, but almost unusably slow with terrible coverage. I chucked away the last Giffgaff sim I had with a lot of credit left on: not wortth wasting time on it.
I've not much recent experience of Vodafone: too expensive.