Parliament will be dissolved on 30th May. When the Commons is dissolved all business is ended and MPs stop being MPs. When the Lords dissolves business ends but Lords still remain Lords.
The government still remains and ministers keep their positions, but is effectively only a caretaker for the pre-election period, and cannot enact new polices except in exceptional circumstances.
The campaign period proper starts once Parliament is dissolved. Parties will publish manifestos shortly after, TV debates may be organised etc.
Yeah you may have heard the term. Parliment is sovereign. It literally means there is never a true lame duck.
While parliment is dissolved. And technically the MPs are no longer MPs. Government can act but only in a clear emergency. The act they can always make. Is to request the king to make temporary laws.
Its never happened since the restoration. But technically the point of our constitutional Monarchy is the king passes power back and forth when parliment is opened and devolved.
In the event russia attacked or something else between 30th May and the end of the election count. Likely 5th or 6th. Sunak can operate government as normal. But would ask the king to enact any change in law. And parliment would be opened soonest once everything is sorted.