Where could you put 7 million people?

I know it’s 3 weeks later, but this point from Dr Who has been bugging me.
Presumably others have spotted this but I haven’t had time to go surfing the net.

In Turn Left, London is destroyed and the South East contaminated so 7 million people have to be re-homed. An army convoy delivers lorry loads of people to a street somewhere up North and Donna and her family end up sharing a tiny terraced house with 20-odd other people. It makes for great drama, but somebody overlooked the maths.

I’ll use round figures to avoid getting picky:

If 7 million need re-housing and 3 million were killed in London, that leaves 50 million people in the UK. Only one person to be re-housed with every 7 inhabitants. With an average of 3 people per home in the UK, the army would only need to squash one extra person in every second home. There are plenty of single people in large houses, loads of holiday homes and even caravan parks. They didn’t need to send them all to one street.

Maths lecture over.