An external audit is a regulated activity and refers to the statutory audit of the accounts for submission to Companies House. An audit is required by statute from all companies over the audit threshold of £5.6 million turnover. Charities must have a statutory audit unless exempt under the Charities Act. You can find out more information regarding the exemptions here
These are the five bodies recognised by the Professional Oversight Board for Accountancy as having rules designed to ensure that auditors are of the highest professional competence:
The Institute of Chartered Accountants, The Institute of Chartered Accountants in England and Wales, The Institute of Chartered Accountants in Ireland, The Association of Chartered Certified Accountants, The Association of Authorised Public Accountants.
It is common for clubs and societies to have it written into their rules that their accounts must be 'audited'. An internal audit, such as this, may be completed by an 'external' person, but it is not the same as a statutory audit and will not be submitted to Companies House.