Like the OP, I'm passing through LHR on Saturday, arriving (I think) T3 on UA and departing T4 on BA for ISB.
My plan is to spend time in the Terraces lounge in T4. Once I get into T4 and the louonge, what sort of security delays will I encounter?
I haven't been through LHR in many years, and I'm wondering what the process if transferring there is like.
Thanks in advance.