Incoming domestic BA flight to T5 connecting onto TATL UA flight. Separate tickets. Has anyone any experience of doing this? Is it quicker airside or landside? Hope to have only hand baggage.

Previously T1 to T3 was a breeze as there was no security just a quick walk to T1 FCC then a bus to T3 but now there is a security check between the Terminals airside to deal with ?

Thanks for any help