Latest update from Affinity support:
We don't have a timeframe to when arrow heads will be added to our Affinity apps at this time. We don't have any mailing lists setup to inform users of fixes. However when we release an update, mail shots are done that include links to the updates release notes, which include information on fixes and features. You can also signup to our support forum and follow the roadmap threads for each app. You'll then receive a notification when a staff member edits this.
I think I will give up on Affinity creating this facility.
Don't give up! As @R C-R mentioned on the first page of this thread, arrowhead line styles are on the current Affinity Designer roadmap. This means that they will be added in a free update to AD 1.x: we just don't know which particular 'x' that will be.
Wow! Affinity Designer looks very good from the git-go. It's fast, well organized and pleasant to use. The text handling has some excellent features already. Scrolling through the font menu and watching selected text change live, quite a treat.
The main addition it needs for me to replace AI is arrowheads. I don't need a lot of useless style options such as Adobe offers, just filled triangles that have some adjustment options.
• placeable on beziers as well as straight lines
• auto scale to line thickness
• user adjustable scale acting on default size (+% and -%, i.e.)
• user adjustable head angle (again, maybe relative to default)
And maybe, if not too much trouble:
• an option to have arrowhead extend line OR
have the line shorten to place the arrow tip at the end of the original segment.
Thanks for listening,
 P.S. Another discussion of arrowheads suggests ability to round corners. Not strictly necessary for me but nice. Acts on all three corners, maybe adjustable as a proportion of the arrowhead's size.