Why there will never be a bridge between Ireland and Scotland
As an exiled English person, the idea of a bridge between Ireland and Scotland makes me happy. Ireland and Scotland launching an economic partnership to stick two fingers up at England is a heartwarming prospect that would benefit them both. It will never happen.
There's no demand. Both Ireland (including The North) and Scotland have very sparse populations and ony about two really major cities. The demand to move cars or lorries between them is very low.
It's the long way. Although Scotland and Ireland are only 13 miles (20 km) at their closest point, that would take you to a part of Scotland that is very difficult to build through. Building a new road from there to Glasgow would be a bigger task than the bridge. The existing roads might be wide enough, but they go such a long way around it would take longer than the current ferry.
The Brits won't pay for it. For the time being both Scotland and Northern Ireland are part of the UK, and this is such a big scheme they would need support from the UK government to get it going. Seeing as it's a vanity project designed to push England out the picture, I can't see Westminster being too supportive of it.
The Irish Sea is unpleasant. I don't know much about ferries, but I've noticed the Irish Sea is always an uncomfortable crossing. I'm reliably informed everybody says this. It would be even more uncomfortable to cross when you're in a car, and even worse if there was an emergency and you had to get out the car. It would probably be closed regularly due to strong winds.
...and the Irish Sea is deep. It's about 300 metres deep with a lot of historical waste thrown down there. It's almost impossible to imagine a bridge support being put down there: it would have to be even longer than that to allow ships to travel underneath. This means it's too deep to tunnel under too.
A train has its challenges. You could offset the costs of a road bridge by building a railway line at the same time. It would be a good excuse to finally improve the Belfast - Dublin service. Problem is Ireland and Scotland use different gauge railways, which would have to be overcome. Again, there is little demand to make the journey: investing the money in Enterprise would be a much more productive idea.
It's very far. You could address some of the challenges above by choosing an easier route that would form a direct line between Belfast and Dubin. The problem is this would be about 25 miles / 40 km. This would make it one of the top five road bridges in the world. Sure, that means you could build it, but why would two countries with lots of housing and healthcare issues invest in such a big vanity project? It's not just building it that would be difficult: maintaining and operating it, bearing in mind there would obviously be no emergency exits, would be a challenge too.
There's still no demand for it. Aside from the politics and jingoism, most of Ireland would benefit more from a faster link to England's big cities, as that's where the goods and people actually go.
So why are they so desperate to build one of these between England and France?
Well, simply put, they aren't. Boris Johnson - who is famous for making headlines and for investing huge amounts of money in crazy ideas that will never be used, has said it would be a good thing to do. Although the English Channel is the busiest shipping lane in the world, which shows there is some demand, the truth is the current rail and ferry service has pleanty of room to carry more people.
In short, the English Channel bridge idea is just a crazy idea to promote himself, where Boris hasn't thought about the practicalities of how it would be built or how it would benefit anybody. He's proposing a giant metaphor for Brexit.