I will give a similar answer to the recent
Excessing a return portion of ticket thread from 2 weeks ago.
This excess is possible, the price is £0.00 as the fare is the same, but good luck getting such an excess, especially a zero one! Knowledge of excess fares is poor among many ticket offices. Some stations are good at it, but many are not.
If you're in this position again, I really would not rely on getting an over-distance excess, and instead buy the ticket to Birmingham New Street and finish short. It's the same price and a lot less hassle. Even if it cost a few pence more, the faffing around isn't worth it!
You do not need to worry about change of
route &
ticket type excesses as if the ticket office can't do them the price is the same on board.
But an over distance excess is a different kettle of fish, and
must be bought before travel (where facilities exist). What is not clear, is if your excess is refused at the ticket office, does that class as having no facilities? If you are refused the excess, then you may be asked to buy an additional single to Birmingham International, you could then write a letter to ask for the money back.