What documents are needed for entry on a UK visitor visa and can I travel with different passports?

The application is online with clear details of what is required

Regarding the passports. I have 2 for business and check out of the departure country (not UK) on one and check in to Thailand on the second, with no issues on departure and arrival. Obviously my DTAC is filled out with the Thailand arrival passport details. When departing Thailand I generally have to show both passports at the departure desk, to show the visa in the other passport. No issues the twice I have done it so far. FYI I wanted my recently acquired Non-O Retirement visa and 12 month extension (and multi re-entry permit) in my new 2025 passport rather that in the other one which expires in 2028.

My suggestion is travel out on the old passport and give that number for your ticket. Travel back with the new passport but have the old passport at hand to show the airline at check-in. 

The passport you use on the airline booking has no connection to immigration. It doesn't matter or dictate what passport immigration will want.

Take both passports when stamping out. I suspect they'll actually put the exit stamp in the new passport

What passport does she have, thai citizens need apply for visa but you say she is not Thai

Depends which passport you used to put on the ticker

Uk immigration website

Common sense....