UK - deportation laws/warnings

[kneal6]

SWIM lives in the UK on an Ancestry visa, he should be a citizen in less than 2 years. Until then, he's unsure what would happen to him if he got caught with a bit of weed by i.e. a police sniffer dog or a car search.

They're supposed to just give out a caution as long as you're not doing something stupid like smoking outside a school.

My question is, would he be deported based on this caution? Is it a criminal conviction? Is it possible to appeal an on-the-spot punishment? Officially he should be deported if he gets any convictions, he just doesn't know what a caution is in the eyes of the law / home office.

SWIM is really wondering if it's a better idea to be buying eighths vs larger amounts.

Thanks in advance for any advice.