BS degrees in the UK are a little different than in the US, 3 years is standard because a lot of what would be first year coursework in the US is covered and credited by their GCSE/GCE exams(somewhat similar to to how you can test out of lots of first year credits in the US with AP tests).

In any case I've never seen anyone with a UK degree have any difficulty having it being equated to a US degree for hiring purposes, and that online CS degree from University of London specifically requires 30 hours of college credit from US students so it still comes to 4 years in the end.