Isn't this part of the 'Blue sky' futuristic idea for a Greater UB I ? Involving four independent countries (assuming a United Ireland) together in a new UBI (Federation structure possibly) with its own written and over arching constitution.
This would likely be a mix of Republics (Ireland and Scotland, presumably being Republics) and (England and Wales) as Kingdoms, each having their own Parliaments/ Sovereign Governments, but also in agreement on a Greater United British Isles [GUBI] operating as a Trading Block.
It seems to be out of the question just now, but with Sein Fein's success in both the North and the South, the chances of a United Ireland look to be possible in the future, although the Republic is unlikely to vote for Unity if it doesn't get financial help to support the North as an enclave of six counties. The SNP are not going away in Scotland, and although the majority at the last referendum was to remain part of the Union, the Brexit break up might shift that balance. In Wales although they want their own language and culture to be highly visible in any new state, economically they would probably want to remain linked to the Kingdom, especially as the heir to the Throne is also Prince of Wales as is a USP in many areas for Wales.
Set against such a future scenario, a bridge linking the two main Islands in the new GUBI doesn't sound so daft. However given all the furore of HS2 even if such a project was feasible in engineering and construction terms, agreeing who would pay for it would bring fresh round of argument.
Better perhaps it stays a potential Boris 'Folly' Idea?