How fast can I tow.

In the UK

On roads where there is a specified speed limit (i.e. 30 mph, 40 mph or 50 mph) then that is the speed you can tow at.

On derestricted roads where normally you would be able to do 60 or 70 mph when not towing, then when you are towing, the speed limit is 10mph less than the specified limit.

So on an unrestricted single carriageway road, which you would normally drive along at 60 mph, without the caravan, you are limited to 50 mph when towing.
On a dual carriageway (with a central reservation) or on a motorway, where you would normally drive at 70 mph, you are limited to 60 mph.
See the Bedforshire Police guide which explains it well.

In France

French law stipulates that, on vehicles (including car and van combinations) with a GTM over 3.5 tonnes, then 80 and 90 stickers must be displayed by vehicles towing a caravan or trailer and these speeds (in kph) stuck to on single and dual carraigeways.
Although, this is supposed to apply only to French registered (local) vehicles, but the gendarme may not always remember this.

Remember that as soon as you pass the sign for the start of a town or village (it's name), the 50kph speed limit applies (unless there is a different indicated speed limit in force).

Check these limits before going aboad. This information can change quite quickly.
Also note that the French have banned you from having speed camera information on your GPS or on maps and that radar detectors ar a big no-no. If either is found then it will be confiscated.

Other counties

See this Wikipedia page on Speed Limits by countries. It includes a column on speed towing limits and enforcement rules.