If they won't stay frozen for 30 minutes, something's wrong, but in that case, they won't be effective so why bother? Perhaps you need a better cooler bag?
Spaking of which, taking a cooler bag does not imply taking frozen gel packs, which you seem to assume it does.
I can't see why they would be allowed, if you could get round the no liquids over 100ml rule by freezing it everyone would be doing it! You are confusing the issue by imagining that because the rule doesn't apply to medicines it might apply to food.
Why not freeze the sandwiches, place in cooler bag, surround with ice cubes in bags from freezer ( so they are at minus 18deg C) , then discard ice cubes immediately prior to going through security?