A buoy that flashes red three times, pauses, then flashes again. Do such things even exist? How would it have been done in Victorian England - LEDs controlled by a microprocessor?
Looks like a lazy way to explain the clue about three red lights.
I've not watched the movie yet (am about to), and don't recall this detail from the book, but I'd say such things at least could have existed.
I'm fairly certain there were lighthouses with distinguishable patterns at the time. You don't need computers for this, just clockworks, and this would be about as basic as it gets for one of those. Compared to music boxes and suchlike, this would be a no-brainer. An old-fashioned red light is just a regular light behind red glass, so no problem there either.
The only problem I can think of is the fuel supply. Light houses had keepers, buoys, not so much. I guess someone could row a boat out to it every once in a while? It doesn't seem all that practical off-shore, but I assume this is a river buoy within London, so... maybe? :)