There are some grade D systems available with call points. They are basically interconnectable detector/sounders and call points. To be honest, it doesn't take much for a company to have these systems described as grade C, all they do is put some form of control panel on the call point unit. (i.e. A test button, a silence button and a reset button).
To answer the questions: Yes, the FRS
can enforce the change from heat to smoke detection. If they consider that heat detection does not protect relevant persons and that smoke detection is reasonably practicable in the circumstances then thay can and should enforce it. (Or alternatively get you to agree to install it without enforcement) It boils down to an argument that has been had many times, and it is whether smoke detection protects the person in the room of origin. (There are many old threads on this matter and I think all opinions are on there so lets not get into it today.
Opinions differ, but the fact remains that a modern system would not be based upon heat detection in the the rooms.
If the FRS are listing a part 1 system with the notice, that cannot be part of the legal side of the notice. All they can tell you on the notice is that legislation you have contravened, and that is what you have to comply with to comply with the notice. i.e. Article 13, The premises is not equipped with suitable detectors and/or alarms. Anything else after that is advice and/or recommendations of one of more method(s) of complying. Many FRS will state a part 1 system because it is the preferred option, and it does the job of leaving the decision to you. i.e. Why should I as an enforcer take the responsiblity for saying a part 6 system is adequate? I will suggest a part 1, if you want to take the responsibilty of installing a lesser system then I won't argue. (I have never heard of an FRS enforcing an alarm system from a pt 6 TO a pt 1)
One final point though, if you install a part 6, I would suggest you do it with FP cabling and adopt the weekly test procedure.