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I do totally agree that the carved log boats were used for river travel only - that seems pretty clear from their design. However, from the half dozen or so boat remains we have, it appears that the sewn plank boats were the pinnacle in British boat-technology in the Early Bronze Age and there was nothing more advanced. We can therefore be less certain about their functions.
Although as you say they could, and may well, have been used for transportation up and down the coast and rivers, the design does not preclude them from sea-going. Ultimately goods were transported to and from the UK from Europe and Scandinavia during the (pre Dover boat) Early Bronze Age and earlier, so clearly they had to be transported somehow - if these sewn plank boats were not used for this purpose, then what was?
Trouble is, we simply don't know the purpose of the Ferriby and other sewn boats. Although they were seaworthy, it doesn't necessarily imply that they were used for the purpose of long-distant trade. Conversely, there is no evidence that they weren't, either.
Goods, and people, were transported to Britain from hundreds of years before the Dover boat. For example, the Amesbury Archer came from the Alps and had a copper knife from Spain and he died roughly 800 years before the Dover boat. I'm afraid the theory that the Dover boat is Britain's first seaworthy boat doesn't, pardon the pun, hold water.