The aircraft was moved to LTN to cover a A321 that has been tech for the last few days and then back to BHX after its rotation.
Now if we were Ryanair or easyjet it would be no problem now would it, we would just cancel the flights and you might get a seat sometime later on in the week if you are lucky.
You get what you pay for, you will 99% of the time get your flight, might not leave on the same day or could be merged with another flight on a larger a/c but it will operate.
So if you want to gamble try Ryanair and now easyjet if reports lately are to go by.