I'm interested to see answers to this but I'd like to say two things.

Firstly the 1983 Labour Party manifesto was a particularly left wing manifesto by the standards of the UK Labour Party. Those who were senior in the Labour party and pro-Europe had left to form their own party, the Social Democratic Party, in 1981 and this left free rein the to the anti-Europeans when drawing up the manifesto for the next election. So I think the text you're provided there needs to be read in that context.

Secondly there have always been Conservative party (that's right wing if you're not British) members who were skeptical of the European project. So prominent were they that there's a term in British politics for them, the "Eurosceptics" . Their numbers have never been sufficient to have any really significant changes made (for instance flat out leaving the EU) but given that Margaret Thatcher would have been considered at least a fellow traveller it's fair to say they've influenced policy to some degree. When Britain refused to fully join the European Exchange Rate Mechanism it was certainly due to Eurosceptic thinking within the Conservative party.

What you're seeing now in British politics is a popular right wing approach gaining visibility where the "person in the street" is engaged with the issues to a significant degree.

EDIT: clarification on scope of comment about manifest being left wing.

If you look at the sides in the 1975 referendum on EEC membership the No side was a coalition of left wing Labour such as Benn and Castle with right wing Tories, especially Enoch Powell. When Thatcher became leader that side of the party gained strength while after 1983 the Labour left was discredited for a generation.

The left was concerned that the EEC would lock Britain into a centrist Europe dominated by German Christian Democrats and French Gaullists. The right feared losing touch with Britain's imperial identity. Under Thatcher the right began to see Europe as a bastion of the kind of partnership economic systems and protectionism they had rejected.

There were also big changes in the EEC (or EU as it became ). Under Jacques Delors's commission presidency there were changes such as the Social Charter which won over the British unions and convinced them that Europe could restrain Thatcherism. Similarly the single currency was anathema to more Nationalist conservative MPs. The view that Europe was a barrier to Britain adopting radical free market reforms grew at each election until under John Major they became a real threat (the so called "Bastard").

Vernon Bogdanor has a good lecture on the referendum here http://www.gresham.ac.uk/lectures-and-events/the-referendum-on-europe-1975

Chris Gifford The making of Eurosceptic Britain has a lot on this.