Lets take the example of the Bombardier contract. If those train carriages were made in Britain it would create jobs in Britain. David Cameron chose the German bid so the jobs will now go to Germany. Importing those 1.5 billion pounds worth of train carriages from Germany will create unemployment in Britain.
Having a huge trade defict of a billion pounds a week means that the above is happening on a very large scale.
(When I say trade deficit I mean the visible trade such as cars, and invisible trade such as insurance added together to make a total trade deficit for Britain).