The lamps seems to be EOL.

It's approaching EOL that's for sure - suppose the QS ballast is helping it to extends its life but if you put it on Switchstart it may basically not work

I still remember several railway platforms that had these lanterns in use when I was young.

There was one railway station on the outskirt village of Reading (Mortimer) and I remember back in my primary schooldays during the 2000s that the platform lighting were these GEC BR railway fluorescents. They were there until like half term October 2010 and luckily 14 year old me at the time managed to get some pics before the Urbis ZX1s with MH bulbs took over. I was like why replace them? Then finding out that these lanterns cannot use 18W T8 tubes due to the QS ballast usage and the wattage configuration rating, is the reason thanks to the phaseout of the T12 tubes that happened around when I was a teen

This is a GEC Z8260, and was acquired as a swap with another collector. Its in stellar condition too. I was told that the ballast in it was originally a 40w ballast for 4' tubes, running two of these tubes in series which would explain why the top tube is in the condition its in. Immediately ripped out, a restored and dedicated twin 2' 40w ballast was installed.

Was the original 40W ballast for 4' tube use a Quickstart one or a SwitchStart? And yeah even though it may sound a good idea to use the 40W ballast for 4'tubes in series for the 2x 20W tubes but that would wear out the tube faster - good job you put in that QS ballast for dedicated twin 2' 40W ballast