The police can only investigate allegations of electoral fraud where there is evidence to show that an offence has taken place. Claims or allegations should always be substantiated when referred to the police.
You should also consider the impact on public trust and confidence of making false or unsubstantiated allegations about the conduct of other campaigners.
Neither the (Acting) Returning Officer nor the Commission regulate these offences. For details of how to report any allegations see our guidance on Reporting allegations of electoral fraud.