Your credit status is only partially determined by your record of keeping up your payments. There will also be a maximum figure which lenders believe that someone with your income will be able to handle.
If we ignore your mortgage for the moment (although, of course, it will actually be taken into account), let's assume that you've got credit cards as follows:
Card 1: £3000 limit. Balance = £0
Card 2: £4000 limit. Balance = £2000.
Card 3: £2000 limit. Balance = £50.
Card 4: £5000 limit. Balance = £0.
When your credit status is being assessed, it won't be the figure of £2050 (that you currently owe on your cards) which is taken into account. A potential lender knows that you can walk out of the door and immediately 'max' all your cards (without needing any further credit checks), up to a debt of £14,000. If the maximum debt which their computer program thinks you can handle (on top of your mortgage repayments) is £14,000 or less, you won't get a single penny of additional credit.
Chris