All lenders must be able to demonstrate what is described as responsible lending. This basically means that they will not lend you money that you cannot afford the repayments on.
Historically they used fixed income multiples such as 4 x your basic salary for example, but this is no longer the case. Under current rules they assess your financial situation and check the affordability of the loan repayments against your existing monthly income and outgoings. This can lead to two people that earn the same money, but have different financial commitments, being offered different loan amounts. The person with bank loans and car finance for example would be offered a smaller loan than the person with no financial commitments.
The other factor that the lender will take into consideration is the loan to value ratio of the mortgage being applied for. They are more likely to offer the same borrower a higher amount at 60% loan to value than they would at 90% loan to value for example. This whole process is described as credit scoring, and means that you can have an excellent credit record but still be declined for mortgage borrowing because you have used your borrowing power in other areas.