BA has been codesharing with Finnair from London to Helsinki for some years but the Finnair prices are often 2-3 times that of the BA flights.
The BA service has been on their usual A320 whereas some Finnair flights are on their A350-900 wide bodied flights.
Anyway everyone with flights from next month onwards seem to have received emails cancelling their flights.
BA lets you rebook on the Codeshares at no charge, or request a refund online. So I have 4 very nice flights with Finnair on a large plane for the two weekends I am travelling.