"Financed independently through member dues (regular monthly direct debit subscriptions), donations, legacy gifts in wills, sales of the Socialist Worker newspaper, books, and literature, as well as revenue from events like the Marxism Festival"
"Members pay tiered monthly direct-debit subscriptions based on their income level (ranging from a few pounds for students and unemployed members up to over £150–£200 a month for higher-income earners)"
I do think an investigation into donations would be interesting. Don't know for sure be would not be surprised if foreign large donations of wasn't found. They do really campaign hard for things that are not in the British publics interest after all.