Indeed, I really don't get why Camelot have decided to move their studio to Pinewood from their own HQ. Why go to all the expense for nothing
As others have pointed out - the draw has never come from Camelot's HQ. Alan has often referred to the studio it has come from as "Lottery HQ" - but that's more a telly-thing.
The draws were originally OBs, then moved to studios in TVC. Then they moved to Arqiva's studios in Buckinghamshire, which are on the BFBS site (effectively a military base - so very secure). However Arqiva's studios there were SD and very small (when they did music numbers on the show they had to pre-record them in the same area that the draw took place in)
Moving to Pinewood as allowed them to go HD (a must now) and given them a more practical space. I've no idea of the resourcing arrangement Endemol (not the BBC - it's an indie) will have had with Arqiva and now Pinewood. If the BBC recommissioned Endemol to make the show, it will no doubt have included HD production in the new commission (it's the default these days) - so that may have meant they had to leave Arqiva?