Unbraked trailers are max 50% of the unladen weight of the towing vehicle (in this case it's the actual weight of the trailer not the plated max weight).
As far as I can recall if you passed your test after 1997 you're limited to the trailer's plated maximum weight being no more than the unladen weight of the towing vehicle. In any case you wouldn't want to tow anything any heavier, and really want to keep to the recommended 85% limit.
Note in both of the above that's assuming the maufacturer's towing limits aren't lower.
I guess part of the problem is you're trike won't have any manufacturer's towing limits to work to. If it hasn't got the unladen weight marked on it you'll need to do so.
Dependant on age the Countryman weighs around 750kg fully laden, so I guess as a guide if the trike weighs over 880kg you'll be OK.
All that said I came across this...
which suggests I'm totally wrong, but I think this applies to the smaller trikes (under 410kg) which are 3 wheel motorbikes, not cars.