I'm also tethering but I've almost used my allowance 😕
Well, from the days I was on BT business broadband, I have my mobile connection still with BT Business, pay only £15 +VAT for unlimited internet (I had threatened to leave), so not 'too' bad (7.5Mbps at the moment), but as I also have servers here, so my domain(s) won't be served until the LOS issue is sorted, and as you say, that'll be with Openjoke.
Regarding the free connection, yes, it WOULD be £70/month if you had 900Mbps, but you don't do you? not sure where you are, but I guess around 100Mbps with the fault, even slower tethering? That would be way cheaper connection, so the maths isn't really that simple.
I just hope that my sudden fault is equally suddenly fixed, but I am really, really surprised that you have been so tolerant with the fault that you have, You don't have to change, just get UnMigrated!
Is FTTP rocket science? I'm starting to wonder......