Don't you think the visas would go to people from countries with less stupid immigration rules? The UK is hardly the only source of educated labour.
Also, there are a hell of a lot of people from the UK who work round the world doing jobs like dishwashing and wouldn't be able to afford to travel if they didn't.
Visa policy for most countries varies wildly according to country of origin.
Most countries operate a tiered approach to visa allocation and often heavily restrict applicants from certain countries if they have disagreements with them.
Is funny that while people from abroad are painted as devious manipulative devils when they are trying to enter the country, they suddenly get portrayed as shining beacons of angelic forgiveness when it comes to the subject of what would happen to all of the British working abroad if we shut our borders.
Also, there are a hell of a lot of people from the UK who work round the world doing jobs like dishwashing and wouldn't be able to afford to travel if they didn't.