From the Electoral Commissioners:
Under the Political Parties, Elections and Referendums Act 2000 (PPERA), there are controls on which donations a political party can accept. These controls apply to parties that are registered in Great Britain. Certain donations must be recorded and reported to us. We publish these reports in a register on our website.

Who receives donations? Donations are made to registered political parties and accounting units (sections of a party whose finances aren’t managed directly by a party’s headquarters). Parties must appoint someone to be registered with us as their treasurer. The registered treasurer is responsible for making sure that the party follows the rules on donations. This includes maintaining suitable systems within the party to ensure donations are dealt with correctly. What is a donation? A donation is money, goods or services given to a party without charge or on non-commercial terms, with a value of over £500. Some examples of donations include: •A gift of money or other property. •Sponsorship of an event or publication.   •Subscription or affiliation payments. •Free or specially discounted use of an office.
Under PPERA, anything with a value of £500 or less is not a donation.

It should be noted that Alastair Campbell’s People’s Vote welcomes offshore donations: