The threat isn't one of invasion or punitive payments, it's simply "if you leave, we're not going to give you preferential treatment in trade or politics". It's

The threat isn't one of invasion or punitive payments, it's simply "if you leave, we're not going to give you preferential treatment in trade or politics". It's kind've self-evident - if you choose to leave the union, why should you still get to play with their toys? If a US state seceded from the union (not that it can), the US wouldn't give it sweetheart deals. If you choose to go it alone, then why shouldn't the union say 'business is business', and do the standard thing in business: hardball. Use your size to squeeze the best possible deal for yourself. The UK 'wanted' out, and that's what 'out' gets you.