If we leave the EU then it is likely that border checks will take place on the Channel crossings in the UK rather than on the French mainland as they are at present - what incentive will there be for the French to stop immigrants in Calais climbing on board lorries bound for the UK.

There would also be the problem of securing the Irish border - whilst Eire is not a  member of the Schengen agreement EU nationals do have the right of entry - currently we effectively have an open border with Southern Ireland which would have to be made secure.  Add to that the possibility/probability of Scotland gaining independence and joining the EU and we have yet another border to secure. 

Resources of the border forces are already stretched - increasing their workload even further is unlikely to help in the task of keeping out illegal immigrants - well not as far as I can see.