it doesn't have to be registered with the dvla you do need to inform your insurance of it, and you do need a certificate to prove it was tested.

it's a pain as you can't go on the eurotunnel, so my regular euro jaunts are done by ferry which is slow

as said, make sure it drives smoothly and powerfully, although expect a slight loss of power. They need to be setup well so that the lpg mixture closely matches the petrol mixture, otherwise the ecu can learn whichever you drive on most and then be rich or lean with not enough adjustment in the lambda correction to get back where it needs to be on the other fuel.

my kit has an oil additive bottle that sucks up a bit of valve lubricating oil and mixes it in to the inlet.

it's well worth having though, my galaxy does the cost equivalent of 40mpg on petrol, which is very good for a vehicle of that size and more economical than a diesel.