As a subscriber to YM and PBO, I note that both have carried articles on the legal requirements to bring in a USA boat to the UK. Something about having it inspected to meet a set of manufacturing requirements. You probably need to contact your Customs office for best info...
(asking strangers for legal help over the internet may pose some, um... risk...)
Late model Ericsons did offer wing/shoal keel configurations according to the brochures, but I am not so sure that very many were sold. I have seen an Ericson show up in the back pages ads in the above two magazines maybe once a year at most. As you note, they are not common in the UK.
FWIW, anything built in California is a rather longish way from you!
It occurs to me that the current weakness of the US$ may bring bring lots more buyers our way... Just like those plane loads of bargain hunting Euro-tourists that enter and leave my local airport (PDX) every day on Lufthansa.
(Since our YC moorage is under the final approaches of PDX, we get all the noise and pollution from many flights a day, including the training sorties from the Air Force base there.

)
The present situation is not so different, really, than our boat-buying situation in the 80's when many of us US shoppers would go to Canada to buy boats "on the cheap," using eighty-cent dollars.
And, yes, I was one of those buyers!
Happy hunting,
Loren