As above, it depends.
If you are looking for originality in a car, particularly if the car is an investment, then matching numbers are important. If on the other hand you want to own a period car and use it as a daily driver or fun car, matching numbers are less important.
Regardless of the intended use, a matching numbers car will sell for more than one with only period correct parts. As for how this applies to British cars... it still applies. All you need to do is look at the number of people who obtain and then reference the Heritage Certificate for their cars.