Why should Brexiteers be at all concerned? It's not as though there's any previous record of perfectly legal EU related referenda being overturned by hook or by crook is there...
We voted to leave the EU and we knew what leaving means. The remainers amongst us might not like it but they really ought to stop telling us we voted for something else and allow HMG to get on with it. There was no provision in the referendum for some form of Brexit proportional to the vote. It was in or out, one side lost. Get over it.