We are Institute of Chartered Accountants in England and Wales (ICAEW) registered auditor. Our registered number is 1234567890.
In the UK most limited companies do not require an auditor for a formal audit of their accounts. 
The Government has produced a straightforward guide, Audit exemption for private limited companies, which explains the position very clearly. 
This is for larger businesses with significant turnover, assets and employees. 
Occasionally smaller entities require an auditor for their own benefit and we can deal with these assignments too.