"It wasn't a devious attempt the sway an election result, it was fraud (to the value of £21.50)."
How does that square with this, then:
"Marsha-Jane Thompson, Momentum’s social media co-ordinator, was sentenced to 100 hours’ community punishment after registering more than 100 fake voters in the east London borough of Newham. Appearing at Snaresbrook Crown Court on 29 March 2006, Ms Thompson admitted filling out and submitting multiple voter registration forms herself, including forging signatures…"
Fraud to the value of £21.50 would not have been dealt with in the Crown Court unless the defendant denied the charge and opted for trial in the Crown Court but Ms Thompson admitted the allegation. Furthermore it would be most unlikely to have attracted a community order with 100 hours of unpaid work.
I cannot see any other reason why somebody would complete multiple registration forms by forging signatures unless it was part of a plot to sway an election result. It depends on your interpretation of the term "petty crime" but in my view any offence which is dealt with in the Crown Court (apart from those where the defendant has opted for trial there and which would otherwise be retained in the Magistrates' Court) can never be described as "petty".