I'm a credit card underwriter for a bank, its very easy to get turned down and its not just based on your credit rating, it depends how long youve lived at your address, been with your employer, residential status, can be a simple thing like you dont have a home telephone number!
When you make an application make sure all the info you put down is 100% correct silly errors can make that decline, it's really stupid i know.
About the voters role that doesnt make a diff if your not on there the company will contact you for proof of address.
It is a good idea to get your credit file to check if there's some thing on there you don't know about. I just got mine found out i got a default i didnt know about!
If all else fails you can appeal the decline and they can look into it further and possibly overturn the decision.
hope this helps!
best of luck, im just glad to have a day off from those applications!