i only see the benefit of staying down south also if you sell and move up later
but then. ie,,if a down south mortgage gets you a 6% rate as the mortgage is bigger but up north its 3% then it balances out a bit.
Also the 'living for now' comes into it a bit
The difference in mortgage rates wont be anywhere near that different though, if there is any difference at all. Checking around a 90% LTV is giving a UK rate of 4.0-4.1, an 80% LTV has a rate of 3.8-3.9% and a 50% LTV at 3.7-3.8%
Other factors will come into play as well, if you have a high paying job they may give you a higher credit rating which would then reduce the mortgage rate.
And all of that is assuming you have the same down-payment regardless of salary, which is not at all expected. If you earn more money then you will find it much easier to save even with higher living costs (and it is always possible to reduce living costs over a time by reducing living standards to save significantly.
E.g., if you lived in the SE you might need 500K for a house but have 50K in down-payment, if you live further out you might only need 250K for a house but will only have 25K down-payment. You are not really any better off on the mortgage.
Ultimately it comes down to your job. As long as you have a job that pays accordingly then you are almost always better off long term (from a financial point of view) living in a higher cost higher salaried area.
If you have a relatively unskilled job that doesn't have a salary that matches living cost differences then yeah, you will be better off living up north in a lower cost area. If you have a relatively non-specialized skill set you will find work across the country, if you have a specialized skill then you will be much more constrained and will more often than not find yourself in the SE.
When figuring finances although having twice the salary wont net you twice the take home pay, your living costs have many fixed costs that don't reduce so your ability to save increases much faster. E.g at 15K it is very to save anything at all, almost everything goes to food, bills, cars, rent. At 30K you can save a little, even if your rent is much higher. At 60K even with high living costs you can save far more. And from what I have seen that is often the salary difference. I've seen jobs in my field advertized at 30-35K in Scotland, but 60-80K in london.
Saying all of that, I could never live in london or the SE!