Well, AFAIK

1 pound = 0.45359237

so 10 stone = 140 lb
and 10 stone 10 lb = 150 lb
so 150*0.45359237 = 68.0388555
rounded to 68.04

That's the result I get in several other web calculators, the result that shg's formula delivers and the result that my suggestion delivers. I'd wager the NHS have it wrong or they're using slightly heavier pounds .