Vince
From reading most of the contents of your link, Statutory Instrument 2014 2080, I would say that an investment firm is a Relevant Financial Institution as defined in 2 (2) b.
However the Bounce Bank Loan Scheme appears to only exclude banks, insurers and reinsurers.
https://www.gov.uk/guidance/apply-for-a-coronavirus-bounce-back-loan#eligibility
Perhaps, an application can be made to a different lender?
Regards
Martin