I think that in UK you have one line of defence/attack and a request for a hand search should be allowed.
The 1971 Criminal Damage Act Section 1 which states:-
Destroying or damaging property.
(1) A person who without lawful excuse destroys or damages any property belonging to another, intending to destroy or damage any such property or being reckless as to whether any such property would be destroyed or damaged shall be guilty of an offence.
The wording in red could come into affect, if clear warning was given to a operator by the owner prior to the start of an examination that you feared that the film would be damaged and that you requested a hand search and this was refused without a reasonable excuse. Not having enough staff to do the task I think would not be a reasonable excuse. Then if they still proceeded and after processing the film was found to be adversely affected, the case would be proven. Not many searchers I think would be willing to take the risk with a maximum term of imprisonment of 5 years being a possibility if they did not heed the request.
WHAT IF IT WAS AN ACCIDENT?
The line between accident and recklessness can be a fine one. To prove recklessness, the court should be sure that you were aware that there was a risk the property would be damaged, and, in the circumstances, it wasn’t reasonable for you to take that risk.