I really am enjoying this thread!
the ##27263# in post #5 did not work for my Boost mobile back here in the UK
I then changed the phone settings with *#*#4636#*#* via the keypad
(as advised in post #18)
to the best of my knowledge LTE/UMS Auto (PLL) is the EU setting
(that's what my HTC One M8 shows)
I cannot guarantee this though as I've not yet got mine unlocked. Boost were a waste of space.
I have not tinkered with TWRP though as I want to keep the phone as original as possible and sell it on ebay.
It did a fine job for me in the USA, but I have no need of it.
the folks linked on post #18 have guaranteed that my they can unlock my phone for only £11.43.
it has to be done via a Windows PC
(probably via ADB?)
so thanks to
@amarc78 and
@uwapalwa there is light at the end of the tunnel!
I'm going to go down that path today and report back later.