similar situation for me arrive this Friday on UA 938 from ORD (est. 1115 but flight has been arriving early around 1050) and then onto 1405 BA flight to BRU from T4(separate tix as my upgradable UA flight was about 450GBP r/t and the BA flight was 38GBP). I'm flying in UA C so I can fast-track immigration (US national, Ms. ORDOGG has UK passport). I think I can use my AA plat for a quick[er] check in (we'll each have a checkin). Should we use the transfer bus or just take the Heathrow express? I understrand it is free to take it from T3 to T4?

thanks!