It's illegal to be in possession of weed in the UK.
For personal use you would probably just get it confiscated and a warning.
For supplying you could get a jail sentence or a large fine.
Both answers so far are, sorry to say guys, slightly inaccurate according to the law. The offence(s) would be committed if you were in unlawful possession of it, i.e. unless prescribed by a doctor etc.