I actually think that the notion of every nation being self-sufficient is ridiculous, and think that creating specialist centres for particular industries according to local resources is a good thing, provided that the wealth and resources are shared equally amongst our species. The problem is that this requires nations to share fairly, which never happens, so we end up with a massively unequal distribution of wealth. Hence we end up with Norway by sheer luck having more wealth per person than we could ever have here. A far better solution would be take a global view of, "We have X people who need Y resources", but I'm not going to pretend that everyone likes each other enough to even make such a world possible (for now, at least). We instead have to compromise, and try to make our situation less bad by building good relationships with those who can help us. To me, leaving the EU would represent turning our back on all the hard work done to build these relationships, and I don't think the outcome would be very pleasant.
If we want self-sufficiency for the UK, we are going to have to do more than just stop immigration. Even if we had total government control of all resources, we have long passed the point of self-sufficiency: we simply cannot sustain our population and quality of life with the available resources from the UK. If we really did want self-sufficiency, we will have to start making a list of things and people we could do without. We have been there before and that is not a world I want to live in, hence I take the view that greater global sharing of resources is the way forward.