There are very few inverts on show at Marwell (or at least there were the last time I visited). In terms of large zoos, Bristol and London are head and shoulders above the other UK zoos in terms of invertebrate displays (though Chester has the best butterfly display - if only they'd put some labels up!).
Marwell's invert collection has gone up a bit compared to what it was:
Baboon Tarantula, Speckled Sun Beetles and Praying Mantis in Heart of Africa
Desert Locusts in Aridlands
Horse-Headed Grasshoppers, Imperial Scorpions, Postman Butterflies, Goliath Bird-Eating Spiders, Fregate Island Beetles and Leaf-Cutter Ants in Tropical World
Partula Snails, Black Widow Spiders, Pink-Toed Tarantulas and MacLeay's Stick Insects in Encounter Village
Giant Stick Insects in Life in the Trees
And those are just the ones on show. However, I don't think Marwell will ever have a strong herp collection, though there are a few gems like the Crocodile Newts and Europe's only Puerto Rican Crested Toads.