Natwest are closing my account after 27 years......

Whatever alarmist stories may be published by the media, I don't think that banks are going around closing long standing main current accounts on a whim. There must be some motive for a particular action. Suspicion of fraud or money laundering. Dormancy. Account misconduct/default. Etc.
The decision to close the OP's account may be nothing to do with the abovementioned events. For all we know, the OP may have been going constantly into unauthorised overdrafts or other such undesirable activity.
Although the OP states that similar activity regularly occurs on the account, that is not in fact the case. On this occasion there were two important differences. Firstly, the money was being transferred to a NEW savings account with another bank. Secondly, on this occasion because the 'battery in the card reader had expired' the OP had to both set up a new payee by phone then make the transfer by phone.
The OP has never stated how much money was in the envelope that the friend deposited. A good question at this point would be to ask the ORIGIN of this money. The OP states 'The paying in of cash and then transfer to my savings account is a regular occurrence and has been for the past few years' but where does this cash come from? This is the OP's main current account which receives their income 'wage goes in / dd's come out ', so where does this cash come from? If as a disabled person you find it difficult to do your own banking, then I am surprised that over the course of many years you have not found a better way of conducting your financial affairs than paying in cash from unknown sources and transferring the cash to other accounts.
I can well imagine that starting a conversation with 'I am recording this call' is probably not going to go down to well with the person at the bank. They are going to instantly assume that the call is a complaint and are at that point probably wishing that they had never answered the call.
You would normally only have needed to use the card reader to set up a new payee. So what you actually wanted to do in this call was not to make your usual transfer at all but instead to set up a new payee, an account with another bank, presumably in your name??? and transfer a (large?) sum of money to that account. A sum of money which had just been deposited in a branch in an envelope. She was probably wondering, as I am, why you bothered. Why didn't you arrange to pay this cash directly into the new account with the other bank? Oh yes. The other bank has no branch in your town. So just why did you open this new account with that bank anyway? Why, after all these years, did you change the destination account of this cash from unknown sources that your are saving regularly?
This really does look suspicious you know. The cash being paid into the account which already receives your wages in envelopes. Transferred out immediately to other accounts. It is actually difficult to quite understand why you choose to conduct your financial affairs in this way.