Also remember anything like old mobile phone contracts, store cards, car hire-purchase schemes etc can all have an impact. Even paying monthly for your car insurance you are effectively in debt to the insurer until the end of the insurance term. If you have any loose ends, tie them up and get things paid off.
Is it the amount of transactions through the credit card or the magnitude of the transactions that counts?
Whilst I'm no financial expert, I would say that it's neither. Quite simply, I think that you just have to prove your ability to spend within your limits, and pay-up again at the end of the month. Whether you're earning £15K a year and able to stay within your £500 credit limit, or earning £100K a year and able to stay within your £10,000 credit limit, the general idea is that you can spend responsibly and pay on-time.
Obviously your annual salary and employment prospects will also have a bearing on how much the bank is willing to lend, there should be some kind of happy-medium where they can judge your risk (i.e. credit score) and ability to re-pay (i.e. your salary) to therefore decide how much they can possibly lend under which terms.