Hi Tony,
As Lutz points out there is often a difference between comfortable and legal.
The legal bit is relatively easy, a car is allowed to tow any trailer up to the the cars stated towing limit - established by the the cars manufacture in the cars specifications. The trailer must conform to the size limits for trailers - but that should not be a problem for UK sourced caravans.
The other legal limit is not set by the car, but on the drivers licence. - Post 1997 UK driving tests only provide the standard driver with Cat B for towing trailers. This limits the driver to outfits where the combined MAM does not exceed 3500Kg, and the trailers MTPLM must not exceed the cars MIRO. Drivers finding they need more capacity have to take the B+E extension test. Pre 1997 tests automatically gave drivers the B+E category.
For an outfit to be legal,it must not be unsafe or dangerous. That may seem obvious,but it is perfectly possible to have a legal weight match that for one of several reasons may be unsafe or dangerous, apart form mechanical faults, issues such as poor loading may make an outfit unsafe.
What is comfortable is the most difficult aspect to predict, its largely down to personal preferences, and experience. Bearing in mind it seems logical to keep the weight of a trailer as small as possible compared to the towing vehicle. I support this view, but unique to the UK the caravan industry suggests you should aim for a weight ratio for the MIRO of the car compared to the MTPLM of the trailer not exceeding 1:0.85 or 85% I must stress that this is only a guideline, and it has no basis is UK law. I and a number of others take issue with the actual figure of 85%, as there has been no evidence provided to back up its validity.
One of the main reasons for my distrust of this guideline is it has been promoted in a dogmatic fashion, and taken on mythical mantle of providing a SAFE towing ratio, and some misguided caravan will religiously work meet this target, whilst overlooking other critical factors that go to make a safe outfit.
There have been posters on the forum that openly admit to speeding, yet would they go over the 85% guideline - never!!!!
In my mind there are many factors that go to make a comfortable and safe outfit,and these include in no specific order:-
Keeping the weight ratio as small as possible.
Achieving the optimal nose weight,
Good maintenance of car and trailer
Correct tyre pressures
and in my view the most important of all is good driving habits.
As for obtaining the figures for cars, there are several web sites that offer towing advice and facts and figures BUT most of them contain errors, the problem is you wont know which are correct and those that are wrong - so beware.
The most reliable source is the cars data plate of V5 registration document, but failing this the car or caravan manufacture.