You make it look like the 'stay' campaign is not funded externally[1]. Obama had openly asked Britishers to vote "stay"[2]. I am not sure about Britain(or the West). But in most of the sovereign nations, this would be considered as foreign interference.

1. https://www.theguardian.com/business/2018/may/29/george-soro...

2. https://www.aljazeera.com/news/2016/04/obama-eu-britain-brex...