321 / Airbridge
As Opsguy says no problem with a 321 on the BA stand. Otherwise there would have been a nasty crunch on the 2 BA 321 arrivals I've been on.
The issue with 757s is wing span. Some time ago the stands were remarked so more props could be squeeze in further along the pier. BA stated at the time the largest aircraft scheduled in future would be a 321.
Don't expect to see BA 757s in future. As terminal 5 at LHR is containerise baggage only, the 757s will be dedicated on routes from T1 (short term), T3 medium term and long term project lauren if it is successful.
CB