Money has no value in itself, it works entirely based on trust, When I buy something from the shop, the shopkeeper may not trust me but he doesn't have to, he trusts money and is confident that he can buy stuff with it from other people because they trust money as much as he and I do. It doesn't matter what form money takes, be it cash, cheque or card. it's the trust that counts. Such is our trust in money that most of it doesn't actually exist. The money deposited in banks actually exists only as a few columns in a balance sheet. This is the greatest scam in history and everyone involved knows it but because everyone believes and trusts in it, it works just fine.
If a Govt wants to take control of all that money it doesn't have to muck about sending troops into the banks, it can just order the banks to hand it over, it's just transferring data after all. What it can't do is force people to have confidence in the new order and once people lose confidence in the great scam that is money it stops working, A pound is only worth a pound because we all agree it is. OK The Govt can force its own people to keep using the pound even though they don't trust them anymore though everywhere this has been tried it has eventually come unstuck.
What it can't do is force other countries to use the pound, We're a lot of people on a small island with no natural resources unless we are prepared to go down the NK route then we will need to buy stuff off other countries and for that we need them to trust the pound enough to take it either directly or swap it for their money OR we have to use their money to buy stuff which means we need to sell them stuff accepting payment in their money for it.
In which case we are limited by how much of their money we have, a problem a lot of Third World especially African countries labour under it.
If the UK Govt wants to nationalise anything especially banks without causing people to lose trust in the scam of money then it has to buy them at a fair price it can't just seize them and there is no conceivable way it can afford this.