Fence Posts

You will need that 2cm of play, you might even need to trim the panels to drop them in.

The +/- tolerance on wany lap panels is huge!

You simply buy tiling batten (which comes ready treated and is cheap as chips) and slip a vertical length down the groove in the post (one per panel is normally enough, but one each side of the panel might be needed if the gap is still too much. When in position run a few screws into pre drilled clearance holes in the fence panel through into the battening. The drilled holes stop the panel splitting. I use screws to reduce the effect of hammering nails in which can break the panel as they are so flimsy these days. When fitted the battens can hardly be seen as they are in the recesses of the post.

Just nail an extra batten or 2 on the edge of the panels?

Indeed. If you've got a table saw, you can easily cut thin slivers off a larger section.

You will need some clearance to get the new panels fitted in the slots of the posts, I know, as I have recently put up a new concrete post fence and panels, as I didn't have just enough clearance on the first panel.

Have you tried other suppliers/measured what they offer? AIUI this is one product where Imperial is still common precisely because of all the posts set at 6 foot intervals. So many panels are in practice a nominal

You can still get 6ft panels - I just did, and went to check when I read your message. Except one which was a tight fit, they all went into the existing posts without difficulty or slopping around. (I used these: