For an offence of causing an unnecessary obstruction, it is common for the courts to require evidence that someone was actually obstructed, rather than they would have been obstructed if they had been there.
Wilful obstruction is a more serious offence, and it is common to require evidence that the owner/driver had been asked to move the obstruction but had refused to do so.
Neither of these requirements seems to be required in law, but they often are in practice. One case from the 1960s involved a vehicle that had not moved for three months. The court agreed with me that this was an unreasonable use of the highway, and was therefore an unnecessary obstruction.