When the UK was in the EU, it secured a huge range of opt-outs, while enjoying all the benefits of the single market, and actively resisted initiatives to further federalize the union.
It was a little more complicated than that: the Single Market was actually Mrs Thatcher's idea. And as that fact demonstrates, the big problem was always a fundamental lack of consistency.
I'm not sure that's strictly true, I think it's more the case that Mrs T's ideas
changed, on the basis of experience (and Delors) once she got into her third term (and got closer to Powell's views on the relationship between the UK and the EU, albeit from quite a different direction).