Maybe contact a few temp controlled hauliers?
Say you are in the early stages of setting up a job and want an estimate for (as you suggest) 4 plts Belgium to London. Id suggest that you ask for their advice on the paperwork involved. They wont want to be held ransom by an outsider getting that wrong.
They might come back with some figures for you. They might not be very interested.
Good luck.
Edit to add:
If it is “common or garden” chocolate such “Snickers” do you really need temp controlled transport?
In high summer or depth of winter sure, but the rest of the year isnt ambient OK? I dont know the regs on this.
Ambient would be cheaper and easier.