I am an Iranian citizen who is going to Cork, Ireland for a short visit with an Irish tourist visa. Can I go to London with this visa or do I need UK visa?
So the Guidance - Common travel area (CTA) claims
certain persons ... who enter the United Kingdom through the Republic of Ireland do require leave to enter.
- persons requiring visas;
The exact same text can also be found in the Immigration Rules paragraph 15. Be warned that the old rules seems not to include this therefore you might get friendly advice which is outdated now! The law has changed in 2014.