Hi,
You're not alone in this type of scenario, and it's not really down to you as an indvidual per se' but more over the algorithm's the various credit reference agencies are using to 'score' you.
So, the first element from me would be to ask if you've checked your own personal credit file and read the report from it, and understood all the various elements it contains and reports about? If you have, can you go through each section of the report and look to see how you can act on the suggestions to help boost your rating. If not, then I suggest this be the first place you look at and check. The one I personally use is CreditExpert -
http://www.creditexpert.co.uk/Another aspect I suggest you can do, is get a pre-paid credit card, and use that and 'operate' the account successfully and properly so that they submit the monthly reports about you to the credit reference agencies.
Do look at your report and act where you can on the suggestions it provides ...
Your bank will also have an internal scoring system, and the same sort of principles apply, have you operated the account sensibly, often in overdraft etc? nowadays the banks may well look at your spending habits and where your income comes from etc - mandatory outgoings, expenses and compare against your income. Put short, if you are spending in what appears to be extravagant choices you'd be most likely turned down for any borrowing. If that's the case, adjust your spending, put more into savings etc.
Borrowing money is much more difficult nowadays - even via the credit card route. Don't be tempted to apply all over the place for lending, the various lending options will refer back to your credit report and say that they've done so so as to leave a trace. Apply for too much credit and subsequent searches will show that up and most likely will refuse you.
See how you go with the above and good luck
Cheers, DtM!