Yes, the idea that only those 70+ are "at risk" is weird. But they probably can't afford to isolate everyone 50+ - both from an economic and herd immunity viewpoint - so they will risk losing 3.6% of people aged 60-69 and 1.3% of people aged 50-59.

According to https://www.statista.com/statistics/281174/uk-population-by-...

8.96 million aged 50-59, 7.07 million aged 60-69.