If an organisation has large numbers of non-indigenous (British) people it suggests to the public that it's not a very good job, or not a nice organisation to work for, this undermines confidence in the fire service & similar organisations.
I don't know if it's the law, or just a guidance. But organisations like the fire service are supposed to reflect the general ethnic make-up of the area they serve: e.g. if 5% of the population are Polish them 5% of the fire service staff must also be Polish. This is ridiculous, because if Polish people want to join the fire service they would've stayed in Poland! If they've got Polish parents, they'd probably apply to join a fire brigade in Poland, where it's cheaper to live.