A rough estimate is to say that you get �100 per week which is 'all yours'. Anything you earn above that amount has one third knocked off in deductions. (22% in income tax and 11% for National Insurance).
As I've said, that's only a rough calculation. It doesn't take into account the 10% tax band which applies to a small part of your income and it assumes that you don't earn enough to pay the higher rates of tax. (It also assumes that you've got a basic tax code). Nevertheless, If you're looking at job adverts and trying to estimate what your take-home pay will be, it's still reasonably accurate.
Chris