I fitted boots in shops (including cotswolds) for a few years. I’m not sure you got the most experienced assistant in the shop there.
Your feet don’t necessarily go up half a size when you stand. Might be more, might be less. I’d go back and get your feet measured again, ideally sitting and standing, or at the very least, just standing.
Get measured in the socks you’re going to wear. Thickness of sock makes quite a difference.
Re. Sizing, most people have different length and shape feet. Nothing odd there. I’d size, for the larger foot. Better to have a boot a touch too big than one that’s too small. Risk of blisters and movement v bruising and loss of toenails. But half a size is about 5mm so you should be able to find a boot that accommodates both.
Meindl used to run about right/maybe a touch large for the size. Subject to measuring your feet properly, I would have put you in the size you measured. Other brands come up differently. They’re also all different shapes and volumes.
Basically, go back, get them to measure you properly and then try a few. (Or just try a few, they’re on your feet after all. All the fitter can do is guide you.) Don’t over think it. If it feels right, it probably is.