do I need the the verification at all in this case if the FAA is going to issue a new certificate anyway when I get my CPL?
Good point. Since a piggyback FAA PPL is dependent on the authenticity and validity of a foreign license, the FAA needs to verify it with the foreign authority. That requires some paperwork exchange with the CAA since they're normally not allowed to release these details to a foreign country. (Data protection act, privacy issues and such.)
The entry requirements for a CPL include an ICAO PPL. I don't think that PPL has to be current in all respects - I think parts of it may be out of date. But I can imagine the FAA would still want to validate the authenticity. So if it's a UK issued PPL, they might still need to contact the UK CAA to make sure it's not a forgery or something.
On the other hand, is it the FAA itself who verifies the entry requirements for the CPL course/exam, or is it the examiner who needs to verify the entry requirements? In the latter case, the FAA examiner might just take a quick glance at your UK PPL and your logbook, and be satisfied you met the entry requirements.
So I don't know whether the CAA validation process is a requirement in this case. Since you're in contact with an FSDO already, I'd ask them about this.