It really doesn't matter what you choose - it will only affect AAISP's ability to pass the buck on who actually supplies the information the government wants.
From reading A&A's wording, I'd suggest it does matter...
If you are a subscriber then the Digital Economy Act means we will have to pass on copyright infringement reports we get about your IP addresses; count those reports;
and maybe take measures to block or restrict your internet service.
If you are a communications provider or service provider
we do not have to do any of those things as they only apply to subscribers.
If you are a service provider you may have to take some actions if you get copyright infringement notices, passing them on to your subscribers. However, if we get such notices about your IP addresses
we do not have to pass them on to you, or tell the copyright owner who you are, or any contact details for you (
unless they have a court order).
So Comms Provider looks like the only one where there is zero likelihood of you receiving notices and/or having to take actions yourself, even under court order.
I'd guess that's designed to prevent (e.g.) BT being dragged into every little episode under normal circumstances? i.e. where EndUser=Subscriber, A&A=ServiceProvider, BT(or BE)=CommsProvider?