copywrite33 :
If you do get accepted, I would suggest you to call them (Flybe's HR department) and try to discuss your situation, that you got a response quite late, and that you have to fly with Flybe and pay 300quid to get to their assessment centre. They are very nice on the phone, and it should affect any of your performance, since the people on the phone are genuinely there to help you. You can then try to discuss and maybe suggest to them to give you a standby ticket.
In the end, you might work for them, so they'll probably consider it seriously.
Hope this helps you.
Best of luck.