We have signed Treaties, and signed up to future commitments which are legally binding. So we must pay them because it is the law. If we ignore the law on this, they might ignore the law in another way by putting huge tariffs on British goods to make them uncompetitive.
In grown up business, extracting the company from contracts takes time, money and patience, but it is worth it in the long run. Getting right now, save a lot of bother later.
Running away is not really the British way of doing things, and even a dysfunctional Government like the one we have got now, will not do anything that reckless.