In years gone by, BA did used to switch to widebody aircraft for routes with major events. I recall travelling LHR - Hannover (for CeBIT) on a 747. Mightily impressed that the crew were able to complete a full meal service (back in the days when you still got a meal in Economy!) and still had time to do a duty free run too!

These days I think BA prefers to keep things as simple as possible and just seeks to optimise its revenue through the mix of premium J/Y fares.