Question for Brits: Can convicted prisoners in the UK wear whatever they want?

Does it differ by locale, in England, Wales, Scitland, N. Ireland? I've seen some pics of prisoners in English prisons and they don't wear a uniform, like in the U.S. All prisoners in the U.S., regardless if state, or whether it's a state or federal prison, wear some kind if uniform. They aren't all the same, some are iust khaki pants, shirts, or jumpsuits of various colors.