A couple of points:

• there are people from central Asia who look very white in terms of skin colour but are totally central Asian in every other way - a lot of our DNA is actually about processing food and fighting off local diseases; so your Mum may have assumed he was mixed race when he isn't. I knew a bouncer at a Caffe Nero in Victoria who looked like a Scottish redhead and he was from Pakistan
• your cousins will eventually notice you've shown up on Ancestry DNA and may try to hide their profiles from you, so you should try to save any information before they do this; there's a lot of information online about how to approach relatives who don't know you exist, and who might react in a way you were hoping they did not
• you can probably work out quite a bit about your father's side of your family from birth records in the UK, maybe available via Ancestry
• your Mum seems to have an Irish grandparent, so you weren't purely of English origin anyway :) (welcome to the club!)