I don't believe that cops can enter your house just on suspicion that you're breaking the law. They do need a warrant for that.

I say this based on personal experience. I've actually had two police enter my house without being invited and without a warrant before; they'd received a call saying that someone had heard a girl being hurt / held against her will (this was not true) - in reality I was sat smoking a bifta with my girlfriend while deep in a Civ 5 game.

Chatting to coppers afterwards (once they'd verified that the call was BS) I queried them on the entry and what, if anything, they would do about the cannabis. In brief their response was:

• You should lock your front door mate - what if it wasn't a copper who let themselves in.
• They have the right to enter without a warrant if they think someone is in immediate danger.
• That they didn't give two fucks about the joint we were smoking in the house, but even if they did the fact that they entered without a warrant to check on someones safety meant they weren't in a position to prosecute anyway.