I suspect many users here frown upon such destructive edit methods since the 'raison d'etre' for Affinity Photo seems to be non-destructive editing. I agree much of the time non-destructive is desirable but sometimes I need destructive techniques.
For anyone interested I worked out a very fast way of achieving a 'crop to selection' as follows:
1 - make a selection with the Rectangular Marquee tool
2 - select Copy flattened
3 - select New from Clipboard
Done.
I was hoping to put steps 2 and 3 in a macro but no go joe! If anyone knows how to put such commands in a Macro please let me know.
TBH, after grappling with this, I should add also that 'Copy Flattened' when used alone actually fulfills a lot of what i was looking for. I'm often copying selections from an image in Affinity into other software and 'Copy Flattened' does the job in the one move i was looking for. So really all this to say that 'Copy Flattened' makes up for not having a 'crop to selection' and in some ways it's better.