We are then free to trade where and with whom and when we like and tailor make trade agreements that suit us and not necessarily the other EU members. The world is opening up and trying to be part of a large block that is somewhat moribund, largely incompetent (can't even present us with an itemised bill for leaving) and as a history of running the most expensive gravy-train in the world, is not the way forward for a large economy like ours.
Yes, I'm talking as much about political boundaries as any trade or physical boundaries. The EU's declared aim now is to move to ever closer integration, to do that it needs to have member states who actually want to be part of a USE, we don't, we will always retain the Pound and will therefore not be running with those in the euro-zone. Britain if it remained in the EU will seek even more opt outs, until eventually with a different currency and loads of opt out's of this and that, we become a side show in the EU, since that is the case I rather we were outside making our own way. The EU will need to get a move on now, to try to restrain China and to try to match the US, both of whom will be vying for the world leadership economy by the end of this century.