Thanks everyone..the responses helped explain what was going on. We did park at a domestic gate I believe on arrival from VIE. I agree that they could have come up with a better set up so that it doesn't matter. Doesn't matter at most airports here in the US. They either deplane the gate at a different level, or manage it with different doors to the direct your path to immigration and customs. It can be done.

I was surprised because prior to T-5, never bussed on connections at LHR..other than to transfer terminals from AA to BA. It was a complication I was not expecting. I now understand it's still a work in progress.

Still a pain...and something I will keep in mind in future trips. Transit over HEL looks better and better if the schedule options work. That said, I for the most part was very impressed by T-5 other than the bus thing. Great terminal and love the F lounge.