For reasons that I can't fathom, the answer to your (very pertinent) question appears to be no. The logic, in so far as there is any, appears to have something to do with the fact that having called a referendum, the government of the day is duty bound to advertise it to the masses. So far so good - nothing wrong with that. Now, as a by-product of leafleting every household in the land, they have the right to outline what they believe to be in the country's best interests. It's as if the government are bipartisan - a neutral public information service - one that's theoretically not in favour of either leave or remain. It's just coincidence that their views happen to tally 100% with those of the official remain camp. So, the £9 million spent on the ad' campaign can not be counted as money spent promoting the remain argument - just as it wouldn't have counted towards the Brexiteers costs had JRM been prime minister and the leaflet outlined the benefits of leaving the EU. I know, I know, its utter bullpoo!
Tim.