Has anyone been prosecuted under the Computer Misuse Act 1990 Section 3A for using or making malware in the UK? I have found example in the US, but none for the UK.
There is an excellent resource for looking at Computer Misuse Act cases:
There have been several cases where people have created DoS tools, botnet software, or credential stealing malware and used them.
For example, web designer Simon Vallor created Gokar, Redesi, Admirer mass mailing viruses that infected 22,000 PCs worldwide. Defendant pleaded guilty, convicted and sentenced to two years jail. BBC news story