We're talking Internet and World Wide Web [they're not "the same thing"!]
There's some useable introductory stuff here: GCSE Computer Science Encryption types
The ICT Teacher Lounge has a few bits: IGCSE ICT - Encrypting Data
Caesar Cypher
This one has some (fairly simple but rather cool) Python code attached: Caesar Cipher - GCSE Computer Science - Compucademy
Symmetric and asymmetric encryption
Some good reading here: Symmetric & Asymmetric Encryption | Definition, Differences
Historical stuff
There's a good piece about Alan Turing (probably the greatest genius behind Colossus) here: The Enigma of Alan Turing - CIA
There's some good stuff about the role of Bletchley Park during WW2, and especially the women involved, here: The Women Who Decoded German Enigma At Bletchley Park
SIGSALY was the first computerised speech encoding system, developed by the Allies (mainly Bell Labs in the US and Alan Turing in the UK). It was the first use of many techniques to deliver encyphered telephony, including PCM, frequency shift, bandwidth compression, FSK-FDM... Check out Vox Ex Machina here; and there's a usable article on Wikipedia.
GCHQ (Government Communication HeadQuarters) has some excellent historical stuff on its Web site, staring with a history of its Origins and WW1 activities for those among you who area just plain fascinated (or die-hard geeks!).
Encryption and codebreaking-related puzzles
The GCHQ Puzzles page Has a collection of codebreaking puzzles, past years' GCHQ Christmas cards (with some serious coding, encryption and more general logic challenges in every one), brainteasers - and they're kind enough to offer a few hints and tips to get you going. If you're working with a friend or a group of friends, you might like to use some of the techniques they share to develop your own codes and brainteasers to challenge one another and develop your skills in encryption, coding, logic, broader thinking...
Brainteasers! There are some FAB puzzles on their brainteasers page when you need to get a bit off topic, but keep those braincells buzzing. Have fun! (... and try to sort them before you check the answers!!)