You will likely have to suck it up and pay for a VPN in order to get a decent enough connection to stream the final. Assuming you’re a UK television license holder you should be able to watch it legally (sort of) through a VPN. I’ve used NordVPN in the past for such purposes.
The free services cap you at a low transfer rate and usually have a monthly cap of 500MB.
Another option might be a free trial of BritBox.
Justwatch says no so there might not be a legit way to stream it. (They're usually pretty good at knowing where you can get things.)
I hope you've found a working solution! If you do need to go with a VPN, I use NordVPN to watch from Canada and iPlayer does not detect that it's a VPN! They likely have a free trial as well. Hope you and your wife are able to tune in!
As an older person, I remember everything in "The Crown" from series 3 onwards.
In 1997 I was in the US and bought Kitty Kelley's "The Royals" most of th
I advise using a Smart DNS Proxy instead of a VPN. It is super simple to do. Its cheap & I use it for this exact purpose.
Then you can watch strictly on iPlayer on whatever you choose: iPhone app / Laptop Web Browser / AppleTV app / etc....