[Making my comment into an answer]
It's not relevant how much you originally paid for the house or how much of your mortgage you have already paid off.
To make it clear in the light of your comment: the equity you have in your old/current house is the current market value minus any outstanding mortgage debt.
From the lender's perspective, they will assume you need
new mortgage = purchase cost of new house - equity in current house.
Then they will look at the new mortgage amount against your salary.
An old rule of thumb for the mortgage was a multiple of 3 times your annual salary, or 2.5 times the joint salary of a couple. Pre-credit-crunch when lenders were competing heavily for custom, this got stretched as far as 5 or 6 times salary.
However this has now been replaced by affordability checks so a better way of looking at it would be to say that your monthly mortgage payment should not be more than about 40% of your net monthly income (after any debt repayment etc. has been taken off). There are plenty of mortgage calculators out there that will take a mortgage amount, term and interest rate and tell you what the monthly repayment will be.
As always with this sort of thing - rather than blindly going with the maximum the bank will lend you, it is worth looking carefully at your own circumstances - what happens if you lose your job (how easily will you be able to get another one?), what happens if interest rates - which are very low and have been for a long time - rise, as they inevitably will.
The other thing to remember is that buying a house incurs expenses over and above the pure cost of the house itself - conveyancing fees, search fees, survey fees, stamp duty etc. Not to mention the stuff you always end up needing when you move house (bits of DIY, furniture, sundries which all add up to a ridiculous amount). So also make sure you have enough left over out of the equation to afford all that.