Hi,
Really poor that you were'nt supplied with a contact list so you should def contact HR and get this.
You can never visit too many units before your interview but to be honest a visit to Edinburgh or Glasgow tower and then perhaps one to Scottish Centre would be enough. Scottish is a bigger operation than Manch anyway and i'll be a lot cheaper to go to Prestwick !! (travel wise I mean)
Although not doing any harm, going to West Drayton or Swanwick won't necessarily gain you any extra "brownie points" at the interview.
Hope the helps
TT