A frames are open to debate and have been for years, with the questions of- is the car then a trailer?, does it need tax/insurance?, do the brakes have to work? blah blah
There's so many opinions (on various forums, mainly reg motorhomes), some say it's just a 4 wheel trailer, then it should have the brakes connected if it's over 750Kg but doesn't need tax etc, some say it's still a car so needs tax, some say it's only legal for recovery work.
I tow the kitten behind my van, no brakes linked but it is taxed/insured. Last year i stopped in a services for breakfast on the way to pod and the dibble were having a right good look at the car behind the van when i came out. A 10 minute chat followed, but nothing to do with towing/A frames etc, just general interest what the car was, what it had in it and how fast it could go!. I think as long as your not being stupid and doing 120mph, i don't think there's any problems.
Don't blame me if you get pulled though!
Ian