Possibly a bit late for you, but a friend swears by Carluccio's!

Have you tried Nife is Life? An Italian online deli with same-day delivery in London. Excellent mozzarella, prosciutto, olives, burrata, bread, plus lots of other cured meats, cheeses, tomatoes etc. nifeislife.com

Does anyone have any recommendations for where I can order a ready made picnic hamper for this weekend? Somewhere local preferably.
Thank you!