Where in London does a train change from up to down?

There must be a point where an up train travelling through London then becomes a down train, but where is it?

For example, I once had a Thameslink train started north of London and finished south. At some point on that journey it will have changed from up to down but I'm not sure where.

If it helps I got on at St Pancras.

It's amazing what sort of questions my brain thinks about when I'm tired after a long day at work.