If we where to go (still not convinced by that) May will not want to use Article 50 and she seems to be favourite.
Could we in theory set up stronger trade and lessen immigration borders between countries like Canada, Aus and NZ etc? As a way of damage control?
You could but:
A: wouldnt that just be considered racist (Kiwi's Aussies and Canadians welcome, all others bugger off).
B: You are not going to replace the European trade value even if you did a deal with the whole Commonwealth and further to that New Zealand at least (I imagine Aussie and Canada are doing the same) is busily trying to negotiate a deal with the EU which is more lucrative for us.
Unfortunately you guys have made your choice and let xenophobic nonsense triumph over rational thought... good luck.