Foreign isn't a race, but under all race discrimination laws in this country, judging based on nationality is seen as racist. As such, you are planning to give an unfair advantage to someone purely on the basis that they are British. Such is against national and international law.
How would you feel if you went for a job in London, knew that you were the best qualified for the job, but were told 'sorry, we don't take any of you lot from Rotherham - Londoners only'?
Discrimination happens regardless of the law, legislating against it is like trying to say "all jobs must be offered on merit and recommendations from managers alone are insufficient". How are you going to prove you didn't get the job because of X anyway? You could say "all the workers are white" and cry racism, but that won't work if 95% of the locals are also white. Statistics would be against your claim.