Further to the above there is a post on the site below which may be of interest albeit the post is over 4 years old. I've no idea whether it would be worth pursuing & I suspect the total length would be over the maximum legally allowed.
"VOSA say any person wishing to tow multiple trailers would need to submit a detailed email to them, they would pass this on to the Construction and Use Team who would then verify the details and determine if it was legal and safe for the public road.
Anyone wishing to should email firstname.lastname@example.org
The Info needed is as follows:
Vehicle Details (Make/Model/VIN Mumber/Reg Number)
Driver Details and License Number (to check entitlement)
Trailer 1 details (length, width, mass, hitch,braked/not)
Trailer 2 details (length, width, mass, hitch,braked/not)
Apparently it is not Illegal so long as the conditions are met under the construction and use regulations.
Fair and circus operators tow multiple trailers as we all know."