UK distributors benefit from that landbridge in the form of cabotage. The less traffic there is, the less benefit of cabotage
I've not seen any empirical evidence that UK distributors benefitted from cabotage.
If anything it was the ROI which benefitted because it reduced the cost of shipping goods there from mainland Europe and being a smaller market the demand for shipment there is lower and it's more difficult to fully utilise vehicles. So what was more likely is a vehicle was travelling from (for example) France to Ireland would be able to drop 30% of its load at places in England or Wales before delivering the final 70% to the ROI - the alternative now is they have to go direct to the ROI at greater cost and have to find a way to utilise the 30% they were previously dispersing in England / Wales.