So UK laws regarding marijuana are much more strict than Australia's.

However, if you're dealing, producing, or trafficking; you're fucked.

So from the information I've gathered, marijuana is completely illegal in UK.

Marijuana is classified a Class B Drug in UK meaning if you're caught with just possessing the drug it's 5 years + fine; and 14 years + fine if supplying.

If you're caught with cannabis for personal use this is what most likely what will happen.

1st time. give a cannabis warning for a first offence of possession – this is a written warning that does not show up on the PCN and should not show on CRB checks often carried out by employers.
2nd time. give a Penalty Notice for Disorder – this is an on-the-spot fine of £80 for a second offence. You must pay this within 14 days or face going to court.
3rd time. make an arrest if it is the third offence of having been caught with cannabis – this could lead to conviction in court and a criminal record.
Every time other the third time, you'll be arrested.

THE POLICE DO NOT NEED TO FOLLOW THIS. IF THEY'RE HAVING A SHIT DAY OR FOR ANY OTHER BULLSHIT REASON, THEY CAN IMMEDIATELY ARREST YOU, EVEN IF IT'S A FIRST OFFENCE.