Definitely, I second the 'how on earth do they even know they will be understaffed at the time of your labour?'. Regardless, you are entitled to a homebirth, unfortunately sometimes it has to be fought for. When my MW mentioned if the MW's on call are with someone else I'd have to come into hospital, I told her that I've planned for that and done my research and the NHS has a service to provide. I also mentioned she'd have to drag me kicking and screaming as I would not go unless there was an emergency with me or baby and I had to, even if that meant my doula ringing for an ambulance, and them waiting outside while I gave birth.
As long as you stand your ground, or your birthing partner does (you won't want to deal with this in labour!) they have to send you a MW, even if it's not someone from the hospital, bank staff etc... Theres a few women who I'm sure will give you lots more info on all the legalities etc.. of this situation, but a good website to look at in the mean time is https://www.homebirth.org.uk/