Having said that, when I have friends visit from Singapore, they often seem to carry £50 notes... and most places seem to accept them (although maybe this is less true outside London).
When I first moved to London my Australian bank changed about $1000 AUD for me to british 50s; I think in that part of the world the banks/money changers get shipments of 50s to save space/weight. Took me a good year to shift those though, 3/4 of places would outright refuse to accept them and the other quarter would handle them with a mix of confusion and distrust.
Having said that, when I have friends visit from Singapore, they often seem to carry £50 notes... and most places seem to accept them (although maybe this is less true outside London).