It's patently obvious to the layperson that the product available at facebook.com in the UK is largely directed by the parent company that is responsible for the facebook.com product globally.

If they're at the "just having a talk" level, they still want to talk to the people who set direction over the relevant parts of Facebook. Which is less obvious, but is still Facebook Inc.

I'll grant that Facebook UK is probably involved with advising the parent company about local laws. However, assume hypothetically that any and all privacy or data protection laws are repealed in the UK tomorrow. Who do you think would set the marching orders for what data is acquired in the UK, and how it is used? Facebook UK?

If Facehook UK existed for more reasons than having a legal entity to handle the UK affairs of Facebook Inc, I'd agree with you. Say, for example, if Facebook UK owned a newstand magazine edited in London and sold primary in England. In that case, if parliament wanted to talk about the magazine then Facebook UK would be the right entity to talk to first.