We have used two options:
1. iPhone personal WiFi hotspot - go to Settings->Personal Hotspot, switch it on and set a password. You can then connect to the hotspot with your computer. We use EE, and it's possible that other providers do not support this. For us this works well and is easy to set up. Good results, even at speed on French and UK motorways, for browsing, email and Skype audio calls. You should switch the hotspot off when you are done, as it uses some battery power to keep it on.
2. Huawei E5785 mobile travel hotspot, with 3 sim, all from Amazon. Slightly more involved to set up, but then you can plug this into the USB socket in the glove box and just leave it there. You need to press a button on the front for a couple of seconds to wake it up, but after that connecting to it is simple.
After connecting the first time, as long as you don't clear the settings on your computer, your computer will probably reconnect automatically to either option.
One word of warning - if you connect an iPhone or iPad to a mobile hotspot over WiFi, they may try to automatically download an update for their operating software. They can't tell the difference between mobile WiFi and your home broadband WiFi. This can be a problem, because updates can be >8 GB in size and if you have a SIM with, say, 12 GB available it will soon be used up.
As long as your SIM provider allows it, and especially if you have unlimited data, the personal hotspot from a phone is probably cheapest and simplest.