The facts of UKIP's offence appears to be:
- A UK citizen and resident, Alan Brown, gave them money.
- They accepted his donation.
- It later emerged that the local authority had omitted him from the electoral register a few months prior to the donation.
- When he found out about his omission he re registered.
- This meant that the donation was made while he wasn't on the electoral register.
Incidentally the amount of the donation is roughly the equivalent to the deposits required to stand in every seat in the UK at the next election.
Note I'm not a member of UKIP and won't be voting for them at the next election, but even so this is an absolutely scandalous decision and hopefully it will be overturned at some point if the Electoral Commission is to retain any integrity.