He did climb Kangenjunga with George Band but not sure they were the first Brits to go 8000m. Certainly, Mallory and Irving had been higher than that although they didn't summit. Nanda Devi had been climbed by then but that's "only" 7817m.
I am unsure whether Mallory and Irvine got to the summit or not. We do know that they didn’t get back down safely. It is possible that Brits got above 8,000 m before Brown and Band, but that doesn’t stop them being the only Brits to be the first to climb an 8,000 m peak, which they did in 1955.
I have a very tenuous link with Joe Brown, the best friend and climbing partner of a late friend and neighbour of mine suffered a head injury in the Alps and was carried off the mountain by Don Whillans. Unfortunately, he didn’t survive and his old wooden shafted ice axe was hung on my friend’s wall for many years.