Natwest's initial response was what you said: his accounts fell below a certain threshold after he paid off his mortgage. Then Farage sought info from Natwest using "subject access request". In those documents, it is "found that an internal committee had deemed his views did not align with the bank's own. This formed part of the basis for cutting him, the document showed, alongside commercial considerations." [1]

[1] https://www.reuters.com/business/finance/natwest-meets-quart...

You are incorrect. Natwest was forced to release internal documents about the debanking of Nigel Farage under an FOI request and it is clear from the evidence that he was debanked because of his politics.

From the Guardian 30 July 2023: <https://www.theguardian.com/business/2023/jul/30/uk-banks-cl...>:

"Farage used a subject access request to discover that, despite initial denials by NatWest subsidiary Coutts, his political views had played a part in the closure of his account."