Undoubtedly, funding of lobbying, formally, informally, secretly, semi-secretly and overtly, is the shape of things nowadays.
So, campaigns are not exempt from this; at 10 weeks out from the date of the referendum the electoral commission will announce the "official" campaign groupings - this is vastly important to them all at present because the designated, favoured, groups will receive public funds for their own particular POV.

As but one more example, here's another case of how funds are allocated to "pressure groups", some of whom are very secret about where the money comes from:-

"This is apparent in the government's review of the balance of competences concerning the environment.807 There, we see the Royal Society for the Protection of Birds (RSPB) expressing the view that the EU's water directives "have been instrumental in delivering improvements in river water quality". The Royal Society also attributed improvements in air quality to EU action and thought that the EU's ambitious climate change targets could provide a competitive advantage over countries which are slower to act.
Yet, nowhere in the entire review is there any indication that the RSPB has been the beneficiary of grants to the value of €14 million from the EU, to support various projects. Nor is there any indication that the international arm of the RSPB, Birdlife International, with offices in Brussels, was lead recipient of funds to the extent of €25,680,683, paid by the European Commission between 2007 and 2012. Furthermore, while the RSPB presents information about itself in the evidence submitted to the review, it makes no reference to its EU funding sources."

Note: this cash came from the European Commission.

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Dave