AFAIK, they are not allowing paid upgrades as such. Skywards Blue members may use miles to upgrade one sector up to 6 hours before the flight. Silver and gold members may use miles for instant upgrades at checkin or onboard, space and catering permitted.

Hope this helps,
^

PS, if a paid upgrade was allowed, you would be looking at a price in the region of £400 to jump from J to F.