Income and details held on your credit file are by far the most important. If you earn a good salary and have a nice, clean credit rating, £5,000 would probably be the starting level if you were to apply for a new card.
Also, if your initial limit is £5,000 and you operate your account responsibly for a year, it is possible that your limit will be raised and then increased again the following year.
Don't forget it's in the Credit Card providers' best interest to force us into a manageable debt situation, which is why credit limits are often both high and increase regularly.
However if you think your limit is too high, it is possible to get it reduced. Just call your provider and ask them to reduce it.