Hi Colin,
Here's a few that I have tried with my piggies & have found out their favourites from that

Spinach, Kale (as a treat), Asparagus, Melon, Green Beans, Cabbage, Pear, Apple, Peppers, Cucumber, Kiwi, Banana, Watercress - trying to think of anymore I've given my girls! My two love the greens the most & the rind from cantaloupe melon.
I think I know what you're talking about with regards to the foraging food - I buy from the natural range at Pets at Home & they absolutely love the herb/flower packs!
With regards to their main diet though, I feed them Burgess excel tasty nuggets with blackcurrant & oregano, which they are a big fan of. The colourful mixed type foods aren't very good for them as they generally pick and choose with them & end up getting deficiency's in certain nutrients.
Hope this helps!