I used travel to Paddington on a Z1-6 travel card, which I used daily
to get from Angel to Barnehurst. When using Heathrow (often) I would
use this card to get to Paddington and then buy a HEX. I am sure I am
not unique in this sense.

When going to Heathrow and I am paying for tickets myself (i.e. not on
business) I take the tube to Heathrow, as I planned to last weekend. I
did spot that the Picadilly line was down on my way there, and that
Z1-6 travel cards were valid on the HEX. I was annoyed, since I went to
Paddington on my Oyster pre-pay (which I had intended to go to Heathrow
with) and then bought a Z1-6 paper ticket at Paddington, so I wasted a
single on my Oyster. I thought I should have been able to use my Oyster
on the HEX given the circumstances. I don't really care about the
technicalities why this may have not been possible - from a passenger
point of view I am railroaded into using Oyster and then later
penalised for using it.