//They arrested over 1000 people last month, so they must of done something//
Indeed they did. But they did it too late and arrest figures do not tell the whole story.
They allowed the principle junction on the premier road in Central London as well of one of the Thames bridges to be occupied by protesters for the best part of a week. The primary function of the police is the prevention of crime. It is not good enough to simply allow it to go ahead - especially when it causes widespread disruption to businesses and thousands of people - for prolonged periods and suggest it was dealt with satisfactorily due to the number of arrests made.
The authorities have had ample notice of this proposed action. They should put plans in place to deal with it swiftly and effectively using the additional powers they have when the target is an airport.
As an aside, I don't quite know why the protesters are worried. They say they will carry out their action unless the government cancels plans for a third runway at Heathrow. As minute as the chance is of the UK becoming engulfed by rising sea levels or the country spontaneously catching fire (or whatever else is forecast to ensue as a result of "climate change") there is a far greater chance of that occurring before a third runway is built.