No harm in booking CAB interview. But be wary of implementing any suggested changes.
CAB are a great free resource. Immigration matters is not their forte.
As Arthur has mentioned, this FLR(M) application is largely a formality as part of the process going from the fiancee visa to extension of stay.
Don't become complacent though.....if your application is not 'straightforward' then paying a premium for a personal appearance at a PEO is not a good strategy.
The PEO application is for those straightforward applications that do not require ANY back-office checking/verification. Means it's not for sorting out issues on the day.
Don't take even the slightest risk of putting yourself in a position of having paid a hefty premium and eventually being loaded at the bottom of the postal applications pile.
No offence intended just trying to help