“Why write a precedent that any clergy could freely dispense to their congregants?”
They set a precedent whether they like it or not. If you just search cheif rabbi goes to church, you can already find religious-left sites using it as a justification for interfaith prayer.
How could they have prevented this? By publishing a psak that makes it very clear that this only applies to this situation and WHY. Otherwise, it’s 100% on them when people misinterpret this.
For the record, as far as I’m concerned, those religious-left sites are internally correct. If the England rabbanut matirs entering a church, then why shouldn’t other people be able to?