I'm not going to recommend a particular place because I've never had the pleasure of breakfast in Watford, but as I said, there are several cafes, bars, eateries at the top of the High Street:
Barracuda Coffee shop, just by the flyover- full English available
Subway - next door
O'Neils - opposite
Nandos - next door
Yates's - not open until 10am?
Jongleurs - Aussie bar
Cafe Toast - opposite end of Clarendon Road
etc etc
There's even kebab houses there if old habits die hard.
Not sure which of these will be open on a Sunday morning though.
If these places seem too salubrious, there is a greasy spoon caff, The Junction Cafe, almost in sight from the station forecourt, on the corner of St Albans Road and Langley Road. That is definitely open from 7am, and comes with a favourable review-
"A great place for a FRY UP!" . This was an opinion on the internet though, so should be taken with the usual pinch of salt, watery pool of ketchup, dollop of mustard, token limp lettuce leaf and as many mugs of strong sugary tea as you can drink.
B&D