According to
Companies House, West Midlands Bus Ltd was founded in June and is owned by West Midlands Travel Ltd. So for whatever reason it looks like Mobico want to retain the legal entity that is West Midlands Travel Ltd, and have (presumably) restructured the business into a new subsidiary whose ownership will eventually change. I am sure there's some excellent legal or accounting reason for doing this but us mere mortals can only guess as to what it is.
I am no expert on these things, but wouldn't that also get rid of the need for Tupe? All the staff get transferred to the new subsidiary whilst under Mobico ownership so there's no real change, then when the sale is completed everything continues as is as the legal entity that is the employer hasn't actually changed?