This is a good question being asked in many places! The legalistic answer is that the whole of the legislation, in particular the Local Government Act 1972, is written to plainly indicate decisions must be taken at a meeting by those councillors who are present. This language permeates all of the legislation.
The practical question is whether this can be ignored in current circumstances. All there can be on this topic is opinion. A definitive ruling would require a court case, and that may not happen soon enough for current needs.
Discussing it with a retired legal officer of a district council, we came to the conclusion that should the issue be taken to court, a judge would most likely rule that holding a meeting using modern digital means was unlawful, but would not set aside the decisions made at such a meeting.
To make the situation completely clear requires legislation, but there are many demands for legislative change at present, and many will probably not be met for some time, if ever.