The Prom 20 refers to the number in the Promenade concert programme held yearly on a variety of classical musical themes , aired on the BBC , so in essence it was a musical concert . They air in honour of Sir Henry Wood the first conductor of the original Promenade concerts that were held in the Queens Hall with Wood as the appointed conductor[and much more] .
It appears that Ben Whitehead was used by Nick Park and Aardman as the voice of Wallace only for a video game they produced called Grand Adventures and was never used in any of the shorts or films, presumably because Peter Sallis had no desire to voice a video game . The classical concert, that was themed on W&G , had a few animated sequences played on a screen behind the orchestra which Ben Woodman voiced but I assume Peter Sallis either was not well enough or declined to feature in the performance .
I think in honour of Peter's contribution they have retired W&G from any feature length films or shorts.