So, there are two issues here. One about working groups and the second about planning procedures.
Firstly, regardless of how a working group is set up/operates, I firmly believe notes should be made of working group meetings. They are not formal meetings and cannot make decisions (which is relevant for the next paragraph) but I have personally found that people's recollection of what has been discussed can vary over time so a note written afterwards is helpful.
Moving on to planning matters, a response on behalf of the council to a planning application is a formal action (even if it is a "no objections" comment). Many councils set up planning committees to meet in between meetings of full council to draw up responses to applications and delegate authority to that committee to respond on behalf of the council. If that committee didn't or couldn't meet, there is no decision possible, so you cannot respond unless you reconvene the planning committee or hold an extraordinary council meeting within the extended deadline for responses.
Some councils have a fall back delegation of authority to the Clerk to respond to planning applications on behalf of the council when the council cannot meet. That delegation of authority needs to be carefully thought through and clear parameters set when the clerk can or cannot use this. For example, it can specify that it can only be used when the planning committee and full council cannot respond within the time limits and restrictions might be placed on applications of a certain size or nature or perhaps applications which are only covered by policy statements within your neighbourhood plan, This is for each council to decide if it choses to do so. Personally I'm not in favour of this approach but I know of councils that use it.