The BBC report that

The Association of Chief Police Officers (ACPO) said any attempts to "disrupt essential goods and services that rely on oil" would be met by "firm action" from police forces.

The Guardian goes into a little more depth:

If the situation deteriorates the government can invoke powers under the Civil Contingencies Act, brought in after the 2000 protests to give police more muscle against protesters.

Once emergency laws to protect or restore the fuel supply are in place police powers could be "limitless", according to the Cabinet Office. In a worst case scenario, this might include calling in troops.
...
A spokesman for [ACPO] said: "We have a raft of legislation to deal with this."

The morning bulletin from ePolitix (I can't find a copy on the site yet) has a slightly more specific take on it:

The Army plans to use heavy vehicles to remove blockading lorries and the government could use anti-terror laws to stop the country being held to ransom by the protesters.

(My emphasis, naturally.)

Exclusion Zones, 2 is a crowd... Has the ability to protest (/argue) under our democracy died out yet?

Update, 15 Sep: A little old to warrant a new post, but also relevant here. Spyblog reports on 6 NO2ID protestors being arrested whilst they drove to a European summit, on "suspicion of conspiracy to commit criminal damage" (my emph again).