Both 244.6 V and 255.9 V are out of spec.
So why post "brief excursions" so sensational?
The supply voltage in the UK is regulated to 240 V, but it can vary up and down a bit depending on the location. In towns and cities the supply is likely very stiff due to massive substation transformers and heavy gauge wiring, and will remain very close to 240 V. In more rural settings where there might be smaller pole mounted transformers and longer runs involved then you can expect more variation.
That said, 255 V would not be very comfortable for 240 V incandescent bulbs. On the other hand, all recent incandescent bulbs I have seen were so called "Long Life" bulbs rated at 250 V. So 255 V would not stress them too much.
For many years, for complicated historical reasons, the standard single phase mains voltage in Western Australia was 250v, whilst in the rest of Australia, it was 240v.
Incandescent 240v lamps made in Australia had very long lives both on their designed voltage & at 250v.
The "new breed" of management came along & decided it was much cheaper to import 240v lamps, & get rid of those pesky workers.
The imported lamps were barely adequate on 240v, but on 250v, were significantly reduced in life.
"All good" quoth the importers, we will import 260v lamps & sell them at ---ahem, a small premium, win-win for everybody!"
The "260v" lamps were a little better, but not much, so soon,"a great bewailing arose in the land".
According to the media & a "tame" EE, it was all the fault of WA's iniquitous 250v system!!
After much agitation, the mains voltage was reduced to a nominal 240v at great cost, but the imported lamps were still crap.
Again, "a great bewailing arose in the land".
Forgetting that locally made lamps lasted much longer, it was all the fault of incandescent lamps, so we must all install compact fluorescents!