In the UK House of Commons, the Speaker can only vote to break ties. The Deputy Speakers do not take part in partisan politics either, and can only raise constituency concerns.
Now, suppose the composition of the Parliament is similar to the one elected in 2017, where the government lost a number of votes. If the Speaker is formerly a member of the party in government, as well as all the Deputy Speakers, won't that cause issues for the party in power? As they lose a few crucial votes which would have come from those members.