As I understand it ... the combination of the Earth spinning, and the magnetic pull between the Poles, means the Earth is slightly squished ... not a perfect sphere.
Soooo ... the Equator is further away from the centre of the Earth, than the Poles are.
And sooo ... if you are travelling towards the Equator, you are getting further away from the centre of the Earth. ie you are travelling uphill.
So, from the North, you go “up” to London.
And, from London, you go “up” to Brighton (which is how it should be).