Date format won't change from US to UK

Hi, I hope someone can help me.

I am a complete newbie to this and trying to learn as I develop my first website and blog, please bare with me and forgive my simplistic knowledge.

I have changed the date and time format in general settings. I have also made sure I have ticked ‘London’ for the time zone too. Despite this, the date still comes up on blog posts as mm/dd/yy, the American version of date formatting. I am less concerned if the time doesn’t appear, although this is frustrating, but I absolutely need the date format to be in the UK style, dd/mm/yy…. well, ideally it would be ‘Tues 14th April, 2015’.

Please help me and be as simple in your explanation as possible, I am still learning and know pretty much nothing about coding.
Much appreciated! Alanna.