I assume the new staff were tested negative before departure but hadn't isolated, and therefore maybe one or two had it, but tested negative. Once in, there's no stopping it, especially if you think the right precautions have been already taken, so you are inclined to live normally. It got into the Falklands this way early on in the pandemic, when we were still getting an understanding.
Since then UK policy for this is quite extensive, I've colleagues who have been working abroad with the military, newly posted personnel go into strict quarantine for 2 weeks at a military base, then they are tested, if negative then they then remain in a bubble to be transferred to their new location, where they stay in a bubble, with testing, on completion of their posting (usually a couple of months), they remain in the bubble until arrival back in the UK, where they then isolate for 10 days at home before returning to their normal job.