I have had afternoon tea at Claridges, but not cocktails. I would say you would definitely not feel a nob in a sparkly dress, its a lovely hotel (with gorgeous loos!) I'm sure you will have a fab time, I'm very envious!
I was a bit baffled by this thread title - thought it would be someone worried about being underdressed, not overdressed! No, you wont feel a nob in the Claridges cocktail bar in a sparkly dress - go for it!