One fact of historical interest is that the Act repealed the Dockyards Protection Act 1772 and thereby removed from the law one of the remaining capital offences - (see debate in Parliament on 12th February 1998).
The 1971 Act section 1 creates the offence of destroying or damaging property:
As with all criminal statutes, application of all the wording is essential. The case law on the interpretation of the Act has been quite extensive. Some of the keywords are 'without lawful excuse.'
There are many instances where an individual may have a lawful excuse for damaging property. However, Section 5 of the 1971 Act deals with two specific instances which might not otherwise amount to lawful excuse: belief in consent - section 5(2)(a) and protection of property - section 5(2)(b).
At Lincoln Magistrates' Court, a District Judge (Magistrates' Courts) has convicted certain defendants of criminal damage - The Guardian 7th October - 'Anti-drones protesters who broke into RAF base are praised by judge.' The Guardian's report indicates the judge said that he had a 'heavy heart' when finding against the defendants and that he 'would welcome an appeal.' The defendants could appeal to the Crown Court where the case would be reheard. The prosecution route for an appeal would be by way of case stated for the opinion of the High Court on a point of law.
The media reports are not very clear about the defence put forward but it seems that it must have been one of 'lawful excuse,' One form of 'lawful excuse' would be self-defence - e.g. where, in self-defence, D hits his attacker with a vase and the vase is broken. Another form would be 'defence of another.' Yet another possibility is that the property was damaged in order to prevent the commission of a crime which would bring into play the Criminal Law Act 1967 section 3.
The Judge is reported to have said:
"I find, and not without some hesitation, that the lack of proximity or relationship between the defendants and those in Afghanistan who may be either targeted or hit accidentally by these drones is insufficient. I therefore, with a very heavy heart, find all the defendants guilty."
In the absence of a full report, it is difficult to comment on the case further. A little further background to the case is at Global Research
An instructive case is the House of Lords decision in R v Jones and others  UKHL 16 where their Lordships held that the word 'crime' in the Criminal Law Act 1967 s.3 did not extend to 'aggression' in international law. A further point is that section 3 only permits such force as is reasonable in the circumstances. Dicta of Lord Hoffmann in R v Jones indicate that the use of force will be severely circumscribed where the individual purports to act in the interests of the community at large as opposed to his own interests. 'The law will not tolerate vigilantes. If the citizen cannot get the courts to order the law enforcement authorities to ... then he must use democratic methods to persuade the government or legislature to intervene.'
The controversial use of drones is the subject of a number of posts on my companion blog Watching the Law - e.g. here.