Audio Research - should I replace the valves?

240v in UK is fine. What they mean is the wall voltage should be at 120/240VAC as the preferred voltage to set bias. This is because the internal amplifier voltage varies if the wall voltage varies. 

So my preamp used 6922’s and they should last about 5,000 - 10,000 hours, but most power tubes should be replaced every 2000  - 2500 hours. 

Real NOS tubes last longer than the current Russian or Chinese production. If you want current production, maybe you could buy Sophia Electric KT88 big bottle. They are better sounding than any KT120.

should I replace the valves?

yes... as many other experienced hands here have said...

Probably a bit of a production from the UK, but when it is tube replacement time on my ARC amp, I ship it to them. They will give the whole thing a once over, test the tubes, and replace as necessary.  They are great to work with and don't keep it too long. And they set the bias. I realize it isn't a difficult thing to do at home, but I don't want to poke around 480V circuits unless absolutely necessary. 

Of course this means no fancy after-market tubes but just what ARC installs.