Thank you to all for your wisdom! Now I have 2.5 hours to transfer at LHR- pleantly of time to get through the e-gates and take the lift back upstiars to check in luggage just for the LHR-IAH segment (I could indeed leave these at checked luggage). Now I've done a same day turnaround similar to this but in GVA and the checkin agent at T5 would not let me check in my luggage at LHR just for the LHR-IAH segment. She insisted I check my luggage to GVA and then recheck it in for the GVA-LHR-IAH flights. Was she wrong?
In any case- I think on everyone's advice, one of the last LON BA services the day before seems to be the safest thing (knew that before) and the 2.5 hour BA-BA connection at AMS seems easy enough if I can check my bags at LHR.