This would do it for you, I think.
The problem with most inflatable trailers is that the tubes sit above the fenders. This means that the boat is really high on the trailer.
Depending on width, you could, I suppose, re-axle the trailer to get enough width to fit the SIB between the fenders. Probably be a little pricey, though.
The Extend-a-hitch would work (depending on the type of trailer you have), but if I were you, I'd print out enough of the pictures that someone on your side of the pond would be able to fabricate one for you, rather than order one from California.