You may believe Faceache broke electoral law in the UK; but they have neither been charged nor convicted. So your belief would be false.
Leave.UK was accused of violating electoral law. I believe the Electoral Commission has now dropped its investigation.
Incidentally, to break UK law, you usually have to perform the violating act in the UK (we have a few laws that involve extraterritoriality, but they are far and few).
Faceache barely does anything in the UK. They apparently make no money here; they don't sell stuff (not even advertising - that would be the Republic of Ireland). CA collected data on US citizens; that's legal here (and would have been of very little interest to Brexit campaigners). As far as I know, nothing CA did violated UK or EU law.
You say "the side they supported won (Brexit)". Do you mean that Faceache supported Brexit? I don't think they had a dog in the race at all. Or do you mean CA? It's been argued that CA took money to provide services to Leave.UK, but that's not the same as saying they supported Brexit.
Trying to "swing elections" is legal in the UK. In fact it is encouraged; people who successfully swing elections often get appointed to important positions such as Prime Minister. It's completely legal to hire polling companies to help you better understand the voters. It's completely legal to hire advertising companies to promote the electoral outcome you want promoted.
Now you are speaking of trust; of course, I trust neither CA (now defunct) nor Faceache. But Faceache did not drag my country into wars in Iraq, Libya and Syria. Faceache did not have that much power. The UK joined those wars largely because of diplomatic pressure from the USA - a much more powerful entity than this rather tedious online advertising company.
From what I have read, and despite their bragging, CA wasn't even helpful to the Leave.UK campaign. Their data wasn't relevant, and anyway Leave.UK didn't have the organisational capacity to make use of whatever data they could offer.