The general rule is that you can borrow between 3 and 4.5 times your pre-tax income for a mortgage in the UK.  If you're buying with someone, 4 times your joint income is a good estimate.  Mortgage calculators online provide varying results.  

For example, the Money Savings Expert calculator tells you if you earn £30,000 a year, you can borrow £97,500 to £135,000, which is in line with the general rule.  The Halifax calculator tells you if you earn the same, you can borrow up to £134,700, so suggests the top end of the general rule.

Usefully, the Halifax calculator asks for more detail than the Money Savings Expert calculator.  This is therefore very helpful if you are paying off loans, credit cards or other debts, as it allows you to consider the effect on what you can borrow.

Both calculators are linked below.  There are many others online - just search "how much will a mortgage provider lend me" or words to that effect.

Health warning!  The online mortgage calculators know very little about you other than what you earn (and the handful of other questions they ask).  Your overall outgoings, savings, credit history, debts, etc will all influence the final decision for lenders.  The figures are very much an estimate for planning purposes.  A Mortgage in Principle (MiP) will give you a firmer understanding of what you can borrow.