This is quite an interesting article on compulsory membership
I have come across some accountants who are not qualified or with the less prominent accountancy bodies but deliberately falsifying accounts for their clients so they do not pay tax. Both parties are happy with the arrangement until client needs to apply for a mortgage.
No doubt there are some very good unqualified accountants but there are so many dodgy ones that are no under scrutiny. I would make anyone who is filing on behalf with their clients should be formally registered with HMRC (not just for Money Laundering) so they are aware and have Professional Indemnity Insurance. Going forward any new accountants should be qualified with certain accountancy bodies. It will not resolve the immediate issues with dishonest accountants but in the long run may raise standards.