If you are going to do any "freelanced" work, you will need to be registered for Money Laundering purposes (it is now a legal requirement!)
- You have the choice - get a practice certificate with the IFA or register with the HMRC.
If your "clients" need an accountants reference, you'd be better off with the IFA practice certificate, as you will not be able to use your ACCA letters.
I understand a number of "time bared" qualified accountants (like you) use this route. At a later date you may wish to be a "registered Auditor", which will require you obtaining your practice certificate via the ACCA.
The IFA practice certificate will give you the experience and make sure you are monitored etc... until you qualify for the ACCA practice certificate.