They're not "common" in any way shape or form, as the Bank of England link above suggests (i.e. there are 10 times as many £20 notes in circulation as there are £50s).
In the scheme of things no, as most people deal with all of their finances electronically where 50's wouldnt come into it.
I am saying that in select industries they are very common, and are usually dealt with in bundles rather than individually. Personally i think they should be updated to reflect the modern security found on the 20.
Question kind of answered though with those number above, because i see them a lot and always have them, i didn't realise the numbers were anywhere near as vast as the link states