Yeah crisps and chocolate really.
Biscuits are a super cheap option, you can buy tons of biscuits for really little cost, and cookie packets are an awesome snack because they tie up with a cup of coffee/tea.
I think like a packet of chocolate bourbons, custard creams, cookies all own brand, they come in clear packets inside so no one needs to know they are cheap! Like 70p and you can split one packet of each per room as they come in strips of 2.
Jammy dodgers & jaffa cakes too.
Pringles are a shout if you go to b&m, if you didn't want to do crisps but they are my favourite snacks.
Could buy a bulk load of pot noodles if you find them on offer. Think that would be about £20 for 30 people.
Flapjacks. b&m do pretty large individual ones for 39p each, and I really like them so do my kids
Could boil a ton of eggs, eggs are great and filling but idk if you'd want the effort of telling people they need eating by xyz
Are families sharing? If so you could actually do the apple/peanut butter jar/loaf of bread. Won't work if it's 30 individual baskets but if it's more 'per room' than per person its a not a bad idea.
Brioche bun packets, I think there is 8 for £1.50
If theres space to cook you could even stick a tin of soup/beans/tuna in each one, they can make sandwiches or beans on toast for snacks too.
Lidl do nice mock mars bars/snickers bars.