Here is a list of some things to look at, this is not an exhaustive list:-
Driving hours and rest breaks, double manning, Allowances for boarding ferries.
Both EU rules/GB rules.
EU working time directive.
Vehicle weights, dimensions, payloads, calculating loads on axles.
Fire extinguisher uses.
Vehicle Dimensions and markers.
Tachograph and rules.
Speed limiters.
Cabotage.
Vehicle Braking.
A bit on maintenance.
Operators licences.
CPC requirements.
What I would say though is that the actual questions turned out to be much easier than the sample questions I found on the site.
You have to read the questions carefully in the exam because sometimes the answer is in the question.
I signed up with a website called LGV Theory. I had to pay a small fee £12.5 but it was really worth it.
I chose that site because it covers case studies and not all sites do.
Anyway, I spent about an hour or two per day reading the training and doing the questions over about a two weeks and I don’t think it would have been possible for me to have failed after doing that because I knew it all at that point, I scored 49/50.