EML, thats very interesting, we have a local company here who do training and testing, last time i enquired (summer last year) i was told that to sit the test you have to hire one of their vehicle/trailer combinations, that was an additional cost of £100 on top of your test cost!!
They wouldnt let you sit the test in your own vehicle

is it a requirement or are you allowed?? Its maybe worth contacting them again...