You'll find charter flight details for all carriers operating from the UK in the winter edition of this:
http://www.airtimepublishing.co.uk/
The BA ski charters were operated in marginal time for the 737 fleet, particularly on Sunday mornings, so not a big impact. There is no such work anyway in the winter period until Christmas (hence all the weekend "Father Christmas" flights in November/December to Finland the holiday charter operators do).