Its something like the weight of the trailer and load must not exceed the full weight of the tow vehicle up to its MAM(iirc) and not exceeding 3500kg.
eg; if your mercs MAM is 1700 and tow limit is 1400 braked it will be fine iirc. However if the trailer you are towing is unladen and plated to 2000kg for example you could be in bother with vosa iirc. That would be as simple as getting it replated to suit.
If you apply it to towing a track car though how many are there that will fit within the limits of most cars capacity for the purpose of our licences? you'd need a 500kg trailer with a 900kg max load. Realistically most will be more than that and not allow much in the way of weight for the load.

IIRC when i looked into it the best combination was a D90 to tow but you'd need to be pretty close on the weight of the track car, mine at the time would have been a bit touch and go with my 205 and plenty of weight saving.

Someone may be along with better advice, i looked into it and gave up for now and will do the test eventually to remove doubt

As with these things i know guys on the same licence that tow all over with play day 4x4s that obviously exceed the weights allowed by their licence but well within the limits of trailers/cars etc.. and have never had any bother. You can look at it that provided its all safe, secure and the trailer fully functioning you can take a chance on not getting a tug but VOSA will tear you a new one in the event of an accident and you're over limits.

I read a story(with a pinch of salt) that a guy was shunted quite seriously when towing a mini digger behind a pickup, VOSA rocked up and despite it not being his fault he still got nailed for being being over weights for his licence..