There are several shops within Cambridgeshire that may be worth trying when trying to buy a trailer.
The most expert company in the area is D-tek near Ely – their speciality is recumbents but we understand they have a good stock of trailers too. http://www.misterwhat.co.uk/company/2412615-dtek-hpvs-ely
Competent bike shops will be able to get stuff for you but are only likely to have kiddie-trailers actually in stock.
There have been a couple of trailer reviews in the Campaign’s newsletters – do a search via the search page. The Bike-Hod is not the world’s best cycle trailer for towing (hitch is too high), but as a compromise between one for pulling around by both bike and by hand it is a good design: http://www.bikehod.com/
It’s much better to have the load carrying built into the bike. E.g. butcher’s bikes and postpersons bikes where a metal frame holds the weight over the front wheel. Another superb design is the long-john which can carry a beer barrel on the low platform between the front wheel and the rider.