Thynker:Ironically, another leading credit agency has me at a 'good' rating, as opposed to the 'poor' one from another
Perhaps that explains your differing experience in getting more credit. You are entitled to be informed which CRA a lender searched.
To turn to the "25 years", I assume that includes time spent in the parental home, how else can 25 years be explained away and still not on the electoral roll?
Anyway the answer is not how long you have spent at an address but how long the CRA(s) record you as having lived at an address. You share responsibility for ensuring that your credit file(s) are accurate and complete.
And there might be another reason. Thanks, or no thanks, to enhanced reporting lenders can see a lot about your credit history. You may say you have half a credit limit left on a card. A lender might see that a balance is not coming down in spite of the income quoted on the application. There is more to credit scoring that just an absence of adverse credit.
I do not know the answers but you will likely struggle until you get on top of your residence and electoral roll history.