Solution of the clue is found!

The clue Sophie found on the gnome was:

"A station today is tomorrow a bridge

Act fast to get to England’s stage."

Solution:

'A station today is tommorow a bridge'

Waterloo Bridge is the location for tomorrow. Because today the location was a station (Waterloo) and tomorrow the location is a bridge (there's a bridge in London called Waterloo, so Waterloo Bridge).

'Act fast to get to England's stage'

The Royal National Theatre is also a location for tomorrow. (England's stage -> a stage in a theatre and the word national refers to England. Act fast refers to acting and that's what takes place in a theatre).

The Royal National Theatre is right next to Waterloo Bridge, as we can see here:



The location for tomorrow is at the Waterloo Bridge, looking at the Royal National Theatre. Lee has just announced in his newest video that the time is 6pm.