Go easy on me as I'm still in Sth Africa, however,will be flying over for Xmas and have looked on line and still uncertain whether I can get to Watford from South East London using public transport.

"Deviance is a freedom enjoyed in a city of lightly engaged strangers"

I can't find any trains running from Euston to Watford Junction on Boxing Day.
I've checked National Rail Enquiries & nothing is advertised.
Southern are running some services into London Victoria.
From there it's looking like the Tube to Watford Underground Station.
It's a 20 minute walk from there.