Do twin HF fluorescent fittings need two good tubes to work or should they
work with one? Or is the answer 'it depends'?

Fitting in question:
https://www.tlc-direct.co.uk/Products/HF236.html

One tube was getting noticeably dark at one end so presumably near end-of-
life. Now neither tube illuminates. Removing the failed tube does not
enable the remaining one to illuminate.

Don't want to get 2 new tubes if the fitting has gone bad, but the sales
blurb says how failed tubes aren't a problem compared to failed LF ones,
yada yada.

If I decide to ditch the current fittings and go for LED strips, some of
the ones available output significantly fewer lumens. To the human eye,
what's the difference between say 5000 lumens and 3500?