The servers are down for the update either way. The enrollment is working but even if you somehow do manage to enrole in the correct number (Which SHOULD be 121 for ALL S20 Unlocked British devices and variants), then you wouldn't be able to download it. I would recommend that you either:
A) Stay in 121 and wait until 3pm for a drop (normal time for updates before the 4pm commuters get home) or;
B) Un-enrole in 121 and wait for the official banner.
I don't know how many slots there will be so I would make sure you get there fast. It is likely, that if they have any plans to drop the OTA download today, then the latest it would be out by would be 3pm (just incase there is a dyer bug which breaks phones, they have support on standby).
Keep an eye out in the following hour ish. If nothing, check tomorrow.