Right now, the marginal price is $36/MWh. Over in England, the marginal price is $268/MWh. [1] When the price of energy is 7x higher, businesses cannot compete. Everything takes energy to use or make. Yet few economists look at the price of energy when deciding which nations will rise and which will fall. Perhaps they should. [1]: https://www.bmreports.com/bmrs/?q=eds/main
You should also consider that the whole idea of a network of large independent system operators is part of the reason why power can be cheap in the US - you can dispatch the most economically viable generation source over a wide area. The UK is roughly equivalent to a mid-sized US state in terms of energy market (roughly half the market of California, for example). Having the UK act as it's own electrical balancing a…
There are some pretty solid geographical reasons why the UK the isn't tied into the European grid (more than by a few interconnects).