You will need two avios award seats in order to book a 241. There is no guarantee that F seats will ever be made available on any particular route. The general advice is to book CW when it is available and continue to watch if two seats are made available in F, then call to change to F.
See these threads:
https://www.flyertalk.com/forum/31910647-post9.html
https://www.flyertalk.com/forum/brit...iscussion.html
https://www.flyertalk.com/forum/brit...os-ticket.html
https://www.flyertalk.com/forum/brit...4-1-first.html
https://www.flyertalk.com/forum/brit...rst-class.html
https://www.flyertalk.com/forum/brit...-vouchers.html