@S M: that Finimus article (“What if my broker goes bust?”) states at the end: “The Financial Services Compensation Scheme covers you for this sort of loss, up to £85,000 per person per institution. So while theoretically the balance of your assets at a broker above this limit ...
M&S Bank is covered by the Financial Services Compensation Scheme. The FSCS can pay compensation to depositors if a bank is unable to meet its financial obligations.