First Choice do a bit of swapping around if any of the B767's become tech. Whenever the allocated 767 is broken, they send a B757 to operate the SFB.
On any normal week no longhaul flights would be operated by a B757.
On another note, notice how all SFB flights are on different days, this is for a reason
Tuesday - East Midlands - 1 B767 free all day and night
Thursday - Glasgow - 1 B757 free all day and night
Friday - Manchester - 1 B757 free all day and night
Saturday - Gatwick - 1 B757 free all day and night