Hi Rob, Welcome to Permies!
Wax Myrtle is on my wants list too. I'm sure it would be an interesting addition to my garden and the thought of a wax harvest from plants is intriguing. I've tried seed twice with no luck,
ART is still the only supplier listed by the RHS and Martin only seems to have seeds listed again. I do have some common Myrtle, which
should provide leaves for flavouring, and bog mytle is a common shrub here. I might try seeing if I can extract wax from those berries, if I can find a female plant. They grow by suckers, so tend to be a single sex thicket.
If you do succeed and have spare plants - think of me please! Maybe we can swap?