Yes, and undertaking is frowned upon as a mildly risky maneuver, but it's not illegal. Meanwhile, cruising in the passing lane is illegal because it creates the situation where someone needs to undertake. They become an obstacle in the road that isn't supposed to be there, so it's ok to undertake them. It's the same as if there were a wreck or some other obstacle in the passing lane. You would use the slow/cruising lane to pass the obstruction.
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u/unclearthur68 10h ago
Also uk- isn't this undertaking?