VERY VERY VERY tough question!
The main fact to consider is that the sausage is THE pivotal point of a breakfast. Bacon, eggs etc, are all foreplay, the sausage is the crux and end all of a breakfast.
So forget the cheap cafes, their sausages cost approximately 1 pence each for them to buy.
I'm going to pitch you a couple of higher end places to get you going, and I'm sure that at some point a row will break out about exactly what constitutes a breakfast. Another thing to consider is, is this breakfast going to be following a night of alcohol fuelled debauchery, a 'who the hell cares about the calories' breakfast or a 'i want the taste whilst trying to be light' breakfast -for the sake of interest I shall pretend the last category doesn't exist.
These are in no particular order:
Vingt Quatre: 325 Fulham Road, well known because its open 24 hours a day, very handy if you're in the area, but also very crowded in the early hours of the morning, Gordon Ramsey sometimes pops in after closing his restaurant with his staff, and if its good enough for him...
Electric Brasserie Its in Nottinghill Gate, don't even think about going there on a saturday or sunday morning as it will be packed with people air-kissing each other, they have the cheek to call it the Full Electric Breakfast, but good it is.
Patisserie Valerie (french chain!) also serves worringly good english breakfasts and has excellent cakes etc to boot.
Simpsons in the Strand is a high end place, which for a little while now has been doing English breakfasts, recommended with a Bloody Mary. Its smart casual and not cheap, but I've heard good things about the quinessintiality (sp??) of it there -I haven't been there (yet)