By ending the Schengen opt out and allowing open borders with the other EU states, we would make border controls at ferry ports and rail terminals (i.e. St Pancras) simpler. Airlines could still demand passports/ID for internal and intra-EU flights as a matter of choice.
Why does this idea matter?
Would speed up intra-EU travel, allowing further integration
Would allow Ireland to enter Schengen rather than having to choose between Schengen and the Common Travel Area
Would allow easier extension of High Speed Rail in Britain by linking any new lines with the EuroTunnel line by allowing EuroTunnel trains to stop at stations without passport controls