Lurked Your comparison with city sizes is slightly erroneous because they don't all go to one place.
But the impact is still the same. You don't somehow stop needing a primary school place for your six-year-old just because you go to live in Sunderland as opposed to London.
I think the problem here is that many people don't quite realise what 336,000 people actually means.
Put it this way, I live in a ward of about 12,000 people. We have eight primary schools servicing the ward with about 56 primary school teachers in total and about 26 TAs. There are an average of 90 births a year. We also have two GP surgeries with a total of 16 GPs and eight registrars in rotation. We also have a secondary school of 1900 pupils.
We have over 6000 cars in the ward with over 5000 people commuting to work every day. There's about an extra thousand who take the bus or train every day. The number of households is around 5200.
If you take my ward as a guide, 336,000 extra people means over 145,000 extra homes. It means over 1500 extra primary school teachers, over 448 extra GPs, over 168,000 extra cars, 28,000 extra people taking public transport. You are talking about 28 extra secondary schools with over 50,000 more pupils.
Going by my council figures, 336,000 people means one and a half new local hospitals, each with a labour ward, a birth centre and substantial out-patients. Looking at the pressure on our local labour ward and birth unit, I'd argue you probably actually need two more of those, rather than one and a half. It's getting near needing another primary trust. You would probably need another two crematoriums as well.
You are probably also looking at 150,000 jobs.
These are serious figures. You can spread the impact around, but, in essence, it's still there. Someone who needs to get the bus to work is still going to need the bus regardless of whether that bus seat is required in Peterborough or Durham. Yes, some areas do have more capacity, but what you tend to find is that that capacity is not as elastic as it first appears or is inappropriate in reality.
For example, we have 2000 empty housing association houses in our borough. But no locals want to live in them -- and neither do migrants or asylum seekers. They are in awkward locations, they are too far from amenities and the town centre, you kinda need a car to get anywhere ... so they stand empty. And then the HA asks what exactly the point of these homes is and "can we knock them down 'cos they are costing us money in security guards and they have high levels of damp anyway because no one has lived in them for so long and we can sell the land to this developer who is going to build houses that people with cars and who don't care about being near to amenities will buy?", we get accusations of demolishing social housing and being in the back pocket of developers.
So everyone gets jittery and the houses are still there. And, eventually, they will fall down. Meanwhile, we have a housing list that is about a mile long.