According to the instructions on my ASDA slug pellet container, the pellets
shouldn't be applied just before rain. It this because here is some danger
of the plants then taking up the toxic metaldehyde and passing it on to
consumers of the crops?

It's because of the pellets disintegrating through being soaked and becoming ineffective.

Keith Cunningham wrote:
According to the instructions on my ASDA slug pellet container, the
pellets shouldn't be applied just before rain. It this because here
is some danger of the plants then taking up the toxic metaldehyde and
passing it on to consumers of the crops?

No: water breaks it down.

--
Mike.