It would have taken some time for my Y-DNA to get across that watery channel from their mainland abode. Trade would have probably started things off, with a few folks risking the sea for establishing exchange with a new and expanding market. I've got some stuff that you might want, and you've got some stuff that I might want, so lets see if we can make a deal..."show me the money!"
I suspect that a few coastal settlements got started. With this new item called "iron", and a new design of the sword, things probably got off to a good start. Over the centuries, trade must have brought increasing numbers of those R1b1b(s) and it would become clear that this island place had something to offer. The Celtic burial rites started to show up, and that two-wheeled vehicle called the chariot started to get placed in grave sites. It would have taken several centuries for the blending of these new cultures, but they were clearly established throughout the island before any of those Latin speaking Romans started to arrive. A Celtic world, scattered about in smaller tribal groups, based upon the family unit. What a deal! At last, my Y-DNA arrived to its new home.
A helpful reference is: "Celtic Britain", by Lloyd Laing, Charles Scribner's Sons, New York, 1979.