No, sigh, it's the other way around. The people who believe the law was broken were lied to, by a corrupt set of regulators. Very apt, given the topic here.

The Electoral Commission spent a long time trying to build cases against Leave campaigners. It did so because although theoretically neutral, like a lot of British government institutions it's run by people who are openly biased and happy to make strongly political statements on the record. The EC is literally run by people who made public statements saying Brexit and the Tories are bad, and that they wanted Remain to win. They also never bothered speaking to the people they were fining as part of their investigation. Not a good start.

https://order-order.com/2018/07/17/electoral-commission-emai...

So they worked over-time to try and prosecute people who campaigned for Leave, but they themselves (fortunately!) do not have law enforcement powers. This is indeed fortunate because every single case they referred to the CPS and Police was dismissed on the grounds that no crime had actually been committed. The High Court has repeatedly criticised the EC for appearing not to know what electoral laws actually say. Or in other words, the people you are claiming "officially found" one side to have violated the law, had their investigations/decisions labelled by the High Court as "unconstructive", "arbitrary" and lacking "any rational basis" (quoting the judgement itself there).

https://order-order.com/2018/09/14/high-court-finds-electora...

Here's another case where the EC lost in court against Leave:

https://order-order.com/2020/04/29/breaking-aaron-banks-wins...

The Metropolitan Police stated the EC itself had broken the law in its attempt to submit a case against Leave campaigners due to withholding evidence and not complying with criminal evidence and procedure law:

https://order-order.com/2019/07/03/met-police-slam-electoral...

Part of the reason the EC's arguments were dismissed is that the EC is responsible for issuing interpretations of electoral law. In at least one case, the Vote Leave campaign requested such an interpretation, was told what they were going to do was entirely legal and OK, and then later the EC changed its mind and decided to fine them for doing it. That sort of thing is hard to explain: either the regulators are corrupt, or incompetent, or both, but it's certainly not the fault of people who explicitly requested clarification about an unclear rule and then were put in Kafka-esque no-win situation.

Naturally they ignored quite blatant violations of electoral law when it was done by Remain campaigners like the famous mailshot to the entire country that mysteriously didn't count as campaign spending, or this one (who they eventually DID fine, but only after a lot of kicking and screaming):

https://order-order.com/2018/08/02/electoral-commission-whit...

The EC's quite transparent campaign against anyone who campaigned for Brexit, certainly motivated by the biases of the people appointed to run it, is a classic case of exactly the same problem Americans are complaining about in this very thread w.r.t. net neutrality. The regulator was captured by a certain viewpoint and appeared to care very little about even the appearance of doing their job properly let alone neutrally.