As you're going to be leaving Bath in the early hours of the morning this makes for a very long day travelling.
As a previous poster has pointed out why not travel the night before and book a night at somewhere like the Premier Inn,a two minute walk from the North Terminal and often reasonably priced.
It removes a lot of the stress and worry about connections.
You could even do what Mrs Clint Bint and I frequently do and get to the Premier Inn even earlier and build in a night on the town in nearby Brighton.
If you're adamant with your plans use the train from Victoria rather than coach.