I doubt that 'nearer 10st' is underweight at 5'8
I'm 5' 10" (might be little less nowadays) - when I was doing more running/cycling/core work in my 20s (I'm now 48) I was about 10st 10lbs, for the last 10-15 yrs I've been a fairly steady 75 kg (11st 11lb), with varying amounts of exercise doing the muscle/bodyfat tradeoff. Currently 32-33" waist (probably 31 if I paid more attention to lovehandles/diet), 39" chest and although I've lost some muscle mass in the last few years from not doing much weights work, you wouldn't call me scrawny.
If you feel you're looking scrawny, mebbe do some weights work to add a little (more useful) bulk, tho' you'll need to eat a bit too..