Yes, taping over a speed camera lens would be illegal.
The UK common law offense of Perverting the Course of Justice would cover (pun intended) this conduct. Common law offenses are not defined by statute (a law promulgated by Parliament or a local government authority), but instead arise from the history of law as applied by the courts.
This secondary source says the offense occurs when one is shown to have:
- acted or embarked on a course of conduct
- which has a tendency to
- and is intended to
- pervert the course of public justice
Other discussions of the offense can be found on Wikipedia, LexisNexis, and many other webpages that may easily be found with a Google search.
Here's a recent case where a driver was imprisoned for three months for fitting his car with a radar jammer that prevented the car from being "seen" by the radar installation.
Thus, if the police can identify the person who made the speed camera ineffective by blocking the camera's lens, that person would be subject to criminal prosecution and punishment.