This might be a reason why we will not have a General Election on the same day as the euro-elections, June 4th.
In the European Parliamentary elections, all spending on behalf of party list candidates is campaign expenditure. They do not have a separate limit or account for individual expenses. As a result, all spending on promotion of the party for the County elections will also be counted against the overall party spending limit for the European Parliamentary election. Expenses incurred on County Council elections to promote individual candidates in those elections are to be reported locally to the returning officer and not through to the Electoral Commission. Campaign expenditure is defined by the Law as spending by a party for promoting the party or its candidates and includes doing so by attempting to damage the electoral prospects of another party or its candidates.
So, the constituency expenditure in a GE would not count against the national totals allowed for the euros. But national expenditure would indeed be counted against those limits, as for the county elections.
So if the GE were the same day as the euro-elections, all parties would be limited to what they are allowed to spend upon the euro-elections only.
And why would Labour constrain themselves so? Thus, at least as far as I read it, no GE on June 4th.
Or maybe it makes it more likely? Knowing that Labour would be massively outspent by the Tories, would they go for it as a way of limiting that outspend?