Like others have said, generally speaking it is best to have a lower utilisation rate, as you are not constantly maxing out your credit, then you are not seen as someone constantly living beyond your means (even though you are repaying it). Of course each lender differs but generally speaking, this is the advice I’ve read all over and to me it makes the most sense. Sure there is a risk of you go out and spend the rest of the £30k but then if your history is only 10% utilisation rate then you have that on your favour. Lenders wants to know you are going to repay the debt and low utilisation rate is normally preferable, also preferable to have a card on your credit line being long standing, it shows you are reliable, loyal etc. Same with the opposite, people who has never had a loan and credit card and never been in debt will find it hard to get a credit card as they will have no credit history. An old card is basically a living proof of a credit history with 1 continuous line of credit.
So I've done some quick searches around utilisation and it is mentioned as being a potentially good thing and a bad thing, lower utilisation can be good, but 0% utilisation can also be bad.
Keeping utilisation around 30%-40% helps show on your credit file an ability to maintain debt and repay. However keeping cards open and not using them is also a bad thing, doesn't help show good management of debt and it does just stack up against you in terms of how much credit a lender will make available to you.
In reality, credit information reported to the credit reference agencies is, balance, payment detail, credit limit. A person using £1.5k on a credit card with a £1.5k limit, but paying it off in full each month, is better then a person with £30k limits, using £1.5k per month, but only paying minimum payment.
Lenders calculate your ability to repay based on income/expenditure, and a credit history that supports repayments. They calculate what they will lend to you based on your total indebtedness. Utilisation is a much lower factor as it cannot be judged in a fair and balanced manner and is really not an indicator of good credit management.
Keeping a big supply of credit availability on old unused cards will have a bigger negative impact overall.
As for keeping your oldest card open, this really needs to be keep your oldest and regularly used card open. A relationship history is what lenders are looking for, and having a credit card that's had a nil balance for 6 years is not a good relationship history. A key consideration point here is that actually a credit history relationship is better represented with long standing bank accounts and mortgages.
