Try your banks? Often easier to get credit at a bank even if you have not had credit with them.
I struggled to get credit because I had never had any. I started using switch to pay for things rather than money from the hole in the wall, and writing the odd cheque and making sure it cashed - little things but does build up your credit reference.
You can't get credit because you don't have credit. You don't have credit because you can't get credit.
The only way I broke the vicious circle was to get a student bank account (they didn't seem too worried) with an overdraft facility and use the overdraft for the sake of using it. Unfortunately that is not possible for most of hte population....
It could be the vicious circle,on the other hand, it could be something simple like they think you're not on the electoral register even though you are (happened to us recently and just took a couple of letters to sort out so don't despair)
Get an Experian report definately as you may have adverse credit that you do not know about, you would be amazed at how many people have problems. Are you on the electoral roll? That helps too. Your bank may be best bet. Have alook at moneysupermarket .com and so on they might be able to point you in the right direction