Don't know how accurate this is, but at the bottom of the page in this link, it lists which flights are affected.
https://www.travelgossip.co.uk/latestnews/agents-and-customers-livid-after-po-cruises-partners-with-one-cabin-airline-with-no-entertainment/
Flight list
P&O Cruises said Maleth will make up around 25-30% of sourcing for Arvia bookings, using two aircraft on turnarounds, one from Gatwick and one from Manchester for Antigua and Barbados. For Britannia, one Maleth aircraft will be used, from Manchester to Barbados, on a Friday only.
The cruise line has released details of the flights that will operate on Maleth airlines. They are:
Britannia: