Even if they catch you with drugs on you, as much as possession (non-dealing) charges have high maximums - for class A drugs I think the maximum is 7 years prison + fine for possession or life in prison for dealing - realistically you're unlikely to get in much trouble for a small quantity. Weed unless you're a serious repeat offender, or a cop decides he hates you, you'll either get an on-the-spot official "caution" and fine up to £90, or more likely a non-recorded verbal warning and confiscation. For harder drugs police aren't meant to do that, but stories I've heard you're still more likely to get confiscation and possibly a caution.
I've even known weed dealers (low level) who've been caught and only had to give up the cash and weed they had on them.
As a slightly related anecdote, a friend of a friend got caught with 11 grams of cocaine (supplying for a party, I think) and by the time he got to court he was being charged with possession of 4 grams, which reduced his "years in prison" with "a few months probation and a fine". No mention of where the 7 grams went in the months between.
In South Korea they can and do arrest and prosecute people for photographic / recorded evidence of them doing drugs even while overseas.