that is easy - I listened to all the 'brexit' (again whatever that is) arguments and it seems that post leaving our allies across the water ((Azincourt, Crecy, those damn Huns (not including Her Maj of course) etc.) there will be no illegal immigrants (like them Roman types and those Normans and of course the Angles), no asylum seekers and of course no refugees, thus allowing the road hauliers association to dispense with having to smuggle people into the country and get back to their traditional ways of contraband booze and baccy hidden within secret compartments presently used for people. this would be a tremendous boost to the economy as you can pack booze and baccy into much smaller places!. You may say this would be a loss to the economy, but this would be offset by the fact that we would need no border control or immigration officers or their non working ever so expensive ships, thus saving millions. it would in fact boost the economy as folk would have more money to spend on important things like bingo and even larger screen televisions.
NOT forgetting the price of postage stamps these days, it means that all them Johhny foreigners are off the Christmas card list thus saving vast amounts!!!!