Okay, well selling class H is definitely upgradable with Avios, so it can't be that.

Not sure what is happening tbh. One possibility is that they've upgraded their IT systems so that the link does not appear if there is no UuA availability, but many here might suggest that sort of change might be rather erm....inconsistent with BA's demonstrated IT capabilities! And if it were such an improvement you'd expect them extend it to cover the case I've shown above, i.e. when the ticket is already booked in the highest cabin for the route. But I suppose it's possible.

Unfortunately, I've only a few upcoming bookings and they are (almost all) in CE, so I don't have much to check against but maybe one of the more frequently flying posters could have a look? I do have one sector in ET and that shows the link for "Upgrade using Avios" despite their being no UuA availability on the flight (but that booking has a GUF2 applied, so that might complicate things)

If you simply need to verify the availability, UuA comes from the same redemption pool as reward flights so you just need to see if there is a straight reward flight available in the class that you are trying to upgrade to, i.e. Club Europe. If there is, you could phone up and do the upgrade that way if you wanted.

However, I don't know what would cause the link to disappear - maybe someone more knowledgeable will be along to help?