Sounds like nonsense to me.
Keep in mind that you don;t need to go to a solicitor to create an EPA (or it;s new replacement, an LPA). It may be advisable to do so, however, if you wish to create a complex one such as multiple attorneys, or one with restrictions or one where you stipulate which circumstances under which it may be activated. Power of Attorney is very powerful, so if you want restrictions or conditions, it is advisable to put them in the correct "legal language".
A EPA (and the new one, an LPA) basically is a document in which someone (called the Donor) grants another person or persons (called Attorneys) to manage their affairs - usually when they become mentally incapable of doing so. They can choose anyone they like to do this. The old EPA only granted powers over financial affairs, the new LPA optionally allows for certain health decisions to be made as well. The Donor may restrict one or more Attorneys in what they can do, and may also stipulate that the Power may only be registered should the Donor become incapable (as otherwise, it may be active right away). The Donor must be "mentally competent" at the time the PA is created or it will be invalid. If this is done by a solicitor, they must be sure that the Donor is competent, and may even seek medical confirmation of this.
Once created, you don't need a solicitor to register a PA of either type - you can do that yourself though the Court of Protection. There are one-off fees, but not anything like £600. Registering means the Power becomes active and once processed by the Court the Attorney gains their powers to manage the Donor's affairs.
You can ask a solititor to store a PA document in their vault, and they might charge you for that although many don't.
Of course, a solicitor may charge whatever fees they like for any of their services, and it's up to you to either accept or decline such services.
I;ve never heard of anything like this "annual fee" though.
I believe that if you want "certified" copies of documents, there may be a charge for these.