I'm sure you've checked out the Financial Requirement for self-employed

If you're a sole trader, partner or franchise, the specified Evidence comprises:-

In respect of self-employment in the UK as a partner, as a sole trader or in a franchise all of the following must be provided:
(a) Evidence of the amount of tax payable, paid and unpaid for the last financial year.

(b) The latest:-
(i) annual self-assessment tax return to HMRC (a copy or print-out)
(ii) Statement of Account (SA300 or SA302)
(iii) the same for the previous financial year if the latest return does not show the necessary level of gross income, but the average of the last 2 financial years does.

(c) Proof of registration with HMRC as self-employed if available.

(d) Each partner's Unique Tax Reference Number (UTR) and/or the UTR of the partnership or business.

(e) Where the person holds or held a separate business bank account(s), bank statements for the same 12-month period as the tax return(s).

(f) Personal bank statements for the same 12-month period as the tax return(s) showing that the income from self-employment has been paid into an account in the name of the person or in the name of the person and their partner jointly.

(g) Evidence of ongoing self-employment through:-
(i) evidence of payment of Class 2 National Insurance contributions (for self-employed persons)
or
(ii) current Appointment Reports from Companies House (for Directors).

(h) One of the following documents must also be submitted:-
(i) (aa) If the applicant’s business is a registered company that is required to produce annual audited accounts, the latest such accounts; or
(bb) If the applicant’s business is not required to produce annual audited accounts, the latest unaudited accounts and an accountant’s certificate of confirmation, from an accountant who is a member of a UK Recognised Supervisory Body (as defined in the Companies Act 2006);
(ii) A certificate of VAT registration and the latest VAT return (a copy or print-out) confirming the VAT registration number, if turnover is in excess of £73,000; (iii) Evidence to show appropriate planning permission or local planning authority consent is held to operate the type/class of business at the trading address (where this is a local authority requirement); or (iv) A franchise agreement signed by both parties.
(i) The document referred to in paragraph 7(h)(iv) must be provided if the organisation is a franchise.
Hope it helps