If you need a driver in the towed car to steer it, the vehicle must be street legal.

If it's attatched to a steerable A frame it becomes a 4 wheel trailer, no tax etc required, you've just got the problem of having something with a suitable towing capactiy to pull it and a licence that covers that combined weight.

A caravan chassis would need to be 4 wheel unless you are only towing a stripped out track car, the suspension won't take the weight of a modern car.
Let alone the chassis which is only designed to carry a big aluminium box.

I've used a caravan based trailer before, they bend.
I've built a car transporter with proper suspension, they are expensive to build.

The cheapest way is to buy a used car transporter trailer,

Actually the cheapest way it to hire one, they are ususally only £40 for a day and you won't have storage problems.

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