you can make it road legal by
working rear lights
brake's on it
seat belts on it
body strething
few other things but there's no proper law on trailers aslong it looks safe police can't stop you
No Proper law??
Road Traffic regulation Act 1984 - Passengers are forbidden in a trailer or caravan except for authorised test personnel while carrying out a test for the manufacturer of the trailer or on behalf of a dealer in trailers.
There is no law governing animals in trailers though
(although it is discouraged in the highway code), so if you wanted to sit a couple of dogs in there that would be fine. I reckon most dogs would love it, going on how they love to stick there heads out of the car window!!