There is work-around for this. I work with a group that also prefers (not mandates) legal names and we have created a simple spreadsheet that we use as look-up to link iNat users and proper names when we import data. It does take effort to assemble but it is very useful. We aren’t trying to be comprehensive, it’s more of an 80/20 approach - getting 20% of users will cover 80% of the observations.
This is 100% contrary to our experience. A surprising number of people include their full names in their profiles or username, at least in my region (Canada). And we will reach out to those with interesting records for their names if they aren’t available in the profile or username. Rarely do people decline, most are eager to be involved. It sounds similar for the UK, so it is possible that they could get a very high percentage of names for UK-based iNat users.