After the nightmare of the second world war - during which we were clearly not self-sustainable and therefore had to rely on food from America (which got torpedoed right the way across the Atlantic and was a nightmare to protect for > 2000 miles) - we deicded for our own sake we had to become self sustainable on the food front.
This was the entire drive behind the agricultural subsidies that exist to this day.
So yes, definitively, in food terms, we are absolutely self sustainable. Not by the free market. By direct government intervention, doling out money to guarantee we stay that way even if the demand is not there. Where we choose to eat stuff imported rather than British foods, we still produce the British foods then the food rots away in barns and sheds (farmers don't care -- government is still buying the stuff) - these are often known as butter mountains.
We keep producing the stuff out of general principle so we can never effectively have the entire country under seige again ..