I just went to the aol.co.uk webite and did "UK only" searches such as "keyword inurl:.com" -- you're right, AOL is choosing to show only .co.uk and .org.uk for their "UK only" searches. Not so Google.co.uk, where any domain extension can be found.
I don't know whether this decision is from AOL or Google - hard to say. But since AOL is the website hosting the search, I'd probably start there with any complaints. Their "Contact" page offers a feedback form for Search.
This one surprises me. On the surface it seems like a real disservice to their users, especially when Google has the data available.