No need to be a Three customer - just go to the following link and order a card:
https://www.three.co.uk/free_sim_mbb/order
Once the SIM comes it needs to be "activated" - I did this by putting it in my phone and connecting to the internet with it, where it took me to a registration/activation page. Once this is done, pop it back out of the phone and then into the SIM holder. You then need to go into settings-communication-car modem (or similar) and tick a box, and your car will then use the boot SIM for data.
The 200Mb automatically renews each month, and if you do use up too much (tip - ensure maps updates automatically is turned off somewhere so you don't blow it all in one update) it simply stops connecting to the internet - no charges whatsoever.
There are other threads on here about the SIM card and accessing the boot SIM holder, which is a bit of a pain from memory as its under the boot floor towards the rear seats somewhere