To be fair the UK is an order of magnitude smaller and election spending hasn’t reached the stratospheric levels of the USA.
There’s also the example of a number of the pro-Leave campaigns in the EU referendum receiving donations from unknown donors, then all spending it with the same strategic communications company behind Ted Cruz’s primary and Trump’s presidential campaigns [1]. In this cases, the sums involved were of the order of millions of pounds — a bit more significant.
Assuming the parties correctly report the spending, or anyone bothers to investigate. I think that is the weak point - if it wasn't for a fairly dogged campaign by Channel 4 the problem with the Tories overspending to promote particular candidates might never have come to light.
For the record, the magic number is £30k per seat contested on "campaign spending" (to promote the national party), and another about £10-16k of "candidate spending" (to promote a particular candidate) depending on the size of the constituency.
That's not really fair at all; given that accounting for size differences really barely nudges the needle - and grey money is likely a factor in US elections too. I can't find a single very trusted source for US spending, but various links (e.g. https://www.statista.com/statistics/216793/fundraising-and-s...) suggest it's been in the billions for quite some time now.
(Not that I'm suggesting the UK is perfect; simply that there is a nevertheless a huge difference)
The headline of that article is that the Conservatives have been fined for it. They were investigated, they were punished.
Whether the punishment is enough or not I don't know, maybe, maybe not, but we are better than the US on this, the US does not have any requirements/legislation about it, let alone levy fines on parties for going over them.
Ignoring differences of degree like that is a sure way to end up in a terrible situation: "it's bad anyhow, who cares if it gets worse?"