They barely observe them. There are workarounds for all these things, and politicians and parties can receive monies by other means. One common trick is to give sweeteners to a politician after they've retired from politics, and the law can't do a thing about it. This is often some kind of directorship of a company, a cushy job in some NGO/ginger group or a lecture tour around wherever. The other way is "treating", where politicians are given luxury accomodation and banquets etc while attending conferences.

I think the main aim is to stop smaller parties from picking up anonymous donations. The major parties in the UK are currently very unappetising to the public (particularly Labour just now, which has one of the least popular PMs ever), so they feel very threatened by the rise of other parties. If the smaller parties such as Your Party (Corbynites), Reform or even Plaid Cymru get enough election funds they can mount a serious challenge.

Worth mentioning that there has been a recent bribery scandal with Nathan Gill of Reform having been given ten years after receiving large amounts of money off Russia to ask questions when he was in the European Parliament.