I think the first line does it. Against socialist principles.
This line doesn't appear anywhere in the document. It does argue that in the 1950's Labour regarded closer integration with Europe would be against the socialist agenda and that any such system would have a permanent anti-socialist majority. It seems that that's as far as you read because the paper goes on to explain that over the next couple of decades many in the Labour Party began to realise that the socialist agenda could be better promoted within the Common Market when they witnessed the appetite among the original six members for addressing living and working conditions. The Unions too were getting on board with a spokesperson from the GMWU observing that
“Joining the EEC will commit Britain to implementing the principle of equal pay." Not something that any socialist would argue against.
He added that it could:
"give added impetus towards longer holidays and shorter hours
lead to an improvement of training methods and facilities.
lead to an improvement in family allowances"
Again nothing to upset the socialist agenda.
At this time, the inhabitants of UK towns and cities which were down on their luck, where the traditional industries were lost or in decline and where there was a widening gap in living conditions, education, infrastructure and healthcare were being advised to uproot and move to wealthier areas, thus abandoning the poorer areas to permanent decay.
In contrast:-
"Another new policy introduced in 1965, in line with existing Labour policies,
was the formation of a committee designed to distribute the money of the European
Development Fund to projects originating in the less-developed EEC Associated
Countries. Funds were provided for infrastructure, as well as public institutions,
such as schools and hospitals."
A socialist policy. So you see, the CM in those days wasn't the capitalist cabal that you imagine it was, but a place where a socialist agenda could be more easily advanced.
We joined the Common Market in 1973 and in 75 the Labour government called a referendum on whether to remain or leave. Remain won with a majority of over 67%. Oh and Labour were the only party to not take a position on either side of the argument, campaigning neither for nor against.