I’ve provided the banks eligibility criteria by means of a direct link to the bank.
Naturally the bank will accommodate clients account fluctuations below that threshold, but it is at the bank’s own discretion.
In the 40 page document you referenced, Coutts clearly stated the commercial decision to maintain Farage’s account while his soon to be mature mortgage was in place, but not to renew their relationship with Farage once the mortgage was finished. (No longer meeting the bank’s financial eligibility threshold).
I doubt that’s an unusual decision for a bank that clearly states it caters for the very wealthy.