I've no idea. I suspect that yes, since it's photographic evidence of what is a crime (possession), whereas if you just tweeted "I'm high as hell right now" that wouldn't be proof of a crime. But I also suspect they'd never bother to arrest for something like that unless you were dealing.

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.