[Grid and UI]

For the past 10 years, I have been designing web user interfaces with a lot of different tools. Affinitiy designer is one of them (and a very nice one!).

 

If you have multiple rows with different number of 'boxes' that, per row, must have the same distance between each box, the approach of using the Align Horizontally (space horizontally) command is the way to go.

However, in what I believe you want to reach, the steps would be as follows:

 

• First roughly place all the rows of boxes with the sizes they should have (equal or unequal, whatever the client wishes), make sure that you place the outer boxes with their 'outside' side on the webpage margin that you what to apply. You might want to use the grid for that or maybe you have already put some guides at the right webpage margins that you can use to snap to.
• Then select all leftmost boxes and align them to their left sides (just to be sure, as they should have snapped to the grid or the guides)
• Then select all rightmost boxes and align them to their right sides (also just to be sure)
• Then select, row by row, all boxes in that row and align them with Align Horizontally (space horizontally)

In that way you get, per row the same spacing between the boxes, but these spaces can differ per row, depending on the widths of the boxes and the number of boxes in a row.

 

Hope this is what you are aiming at.