A wise and thoughtful post, mikes. So many good points in it, but this is a key one which is not mentioned nearly enough:
we will have a badly wounded EU on our doorstep
Nearly all of the national debate revolves narrowly around what is best for the UK. Rarely does anyone mention what might be best for the EU. But if we leave, the EU will still be there, and whatever happens there will profoundly affect us, whether we are members or not.
We live in uncertain times. The rise of extremist parties in Germany, Poland, Spain and France and elsewhere, combined with the ongoing Euro crisis and the flow of refugees and other migrants from the middle East and Africa, are massive problems which will not be solved by our departure. On the contrary, if anything we will just be making them worse!
Virtually every country in the world (apart from Russia, of course, which has its own agenda) is worried about the destabilising effect on peace and security that Brexit would have. We are a force for good in the EU, and our presence is sorely needed. We should not forget that.