I know at LHR, BA seem to disembark from Door 1L and reposition the bridge to 2L for boarding if there's only the one.
Not always. Been on plenty of mid-J B747's (where WT+ is behind F) and the jetbridge has been attached to 2L. Crew either try their best to keep WT+ seated and get F off first, or, admit defeat and just let WT+ off as quickly as they can followed by F (so, not a bad perk for those in WT+ really).