Clear answer to that is the Marriott. It is a standalone building sitting between the B and C terminal. There is an underground mini tube/subway loop system (this is landside) and the hotel has its own stop, which means you get on at the A terminal (which AA uses), and get off at the second stop for the hotel. Later on you continue in the same direction, and again second stop for BA's D terminal, after the C terminal. This train system consists of smallish buggies and runs free of charge every few minutes.

I've never stayed at this Marriott, so I don't know if it's any good or not,. but it's clearly visible when arriving at IAH.

To ease AA with the baggage (who normally have few problems with inter-lining) you best have both locators clearly written out, and both ticket numbers visible too. You can get this via classic.checkmytrip.com

Also, with so much time between flights, don't think it immunises you from delayed baggage - sometimes it adds to the risk. So make sure anything you really need is in hand luggage.

I guess you will be aware that there is an earlier LHR plane from IAH.