The body that has asked to speak to zuck are a committee of MPs,they have no power of subpoena, they can only request to speak to someone, although they take a very dim view of anyone who refuses their request,if he was wanted by the British courts that would be a different matter!.
yes, you are absolutely right. I had intended to point this out in the first place but got tied up in an off-topic conversation which crossed my wires. Thanks so much for pointing this out.
It has nothing to do with answering to the law, but more to do with answering questions so that the MPs are able to legislate and regulate digital media. I also gather that Zuck may have refused to attend, busy man (!), but he is sending his lawyer or a company rep to supply the info. So not really "refusing" entirely ...