Modern Glass Reinforced Plastic (GRP) Mirrors are built from a GRP & foam sandwich construction (i.e. a layer of GRP, a layer of foam, a layer of GRP) sometimes called Foam Reinforced Plastic (FRP). This produces a hull that is lightweight, normally around the minimum permitted hull weight of 45.5kgs, and stiff, so the hull panels don't bend much when the boat is sailing fast in waves and the boat won't distort much if you have a lot of tension in the rig (shrouds & forestay).
However, the outer layer of GRP is quite thin, so the construction is quite fragile in some ways. If, for example, you rested the boat on a pebble on a beach, or bumped the topsides on a jetty, it would almost certainly leave a depression mark in the hull. The same is true of both the Winder GRP and Trident GRP boats since they are both foam sandwich.
That said, I know GRP Mirrors have been built for Scout Groups and similar, so it would be well worth talking to the Licenced GRP builders to see what they have done and could do to make a boat more robust.
If you are intending to sail such that you will need to drag the boat up the beach (e.g. cruising or camping) you might be better to go for a good condition wooden boat that will not require too much maintenance & would stand up to this better.