Advice from my bank manager has lowered my credit score

On the subject of old accounts, I have a 30+ year old pre-1988 current account with overdraft facility which is reported normally on my credit files and has been reported for as long as I have been checking my files. I have never taken action to get this account reported.
I believe it would be beneficial to your credit files to try to get your account reported. The obvious benefits are that the age of the account will both show current account stability over a long period and increase the average age of your accounts which you say is very poor, and also show that you actually have a current account and are trusted with an overdraft facility. As it stands, any potential lender checking your credit files may draw the conclusion that you keep your money under a mattress. The existence of a good overdraft facility, especially if never used, can also give lenders an extra degree of confidence regarding your ability to make on time payments. Does this account actually have an overdraft facility? If it doesn't that is the obvious reason why it is not being reported. Try to get an overdraft facility and it may well start reporting automatically. If you already have an overdraft facility, talk to the bank and see what can be done to get the account reported. I don't quite understand how this new negative concerning the age of your accounts has come about because the closed accounts will not have disappeared from your credit files. They will still be there for the next six years as closed accounts with their ages still showing.
I find it very strange that your bank was offering you a pre-approved unsecured loan for £27,000. This is a very high amount and somewhat disproportionate to the £2000 credit limit that you had on your one and only credit card. Of course, pre-approved probably means nothing of the sort but still the amount almost defies belief as does your claim that when you logged onto online banking it was actually written there that you were no longer pre-approved for a loan as they did not know enough about your income and expenditure. I have never previously come across an example of a bank stating a reason why a customer was not pre-approved for something.
As to what you should read into the fact that you now once again have a pre-approved offer, albeit for a lower and far more realistic amount, is perhaps that the bank does now know enough about your income and expenditure. Are you sure this loan is actually pre-approved or does it say you are pre-approved to apply for a loan of up to the amount stated.
Regarding the current use of your credit card. using the card and repaying in full every month is of course the best thing to do, but there is absolutely no need to repay it two days after the statement date (unless that is convenient for you of course). It would be more normal to set up a direct debit for the full balance and just let it go through on the payment due date. Your credit files will not show the date of payment. I would also suggest, if realistically possible, trying to get the original credit limit reinstated or at least increased to over £1000, preferably more.