We did not negotiate perks, we simply refused to move from our existing conditions when new treaties were negotiated. i.e. we stayed with something close to o

We did not negotiate perks, we simply refused to move from our existing conditions when new treaties were negotiated. i.e. we stayed with something close to our entry conditions, accepted some changes from new treaties. So the "opt outs" were really declining to "opt in" to new treaty terms. Any new entrant to the EU has to accept the whole Acquis as it is at the time. So if the EU would have us again, we'd be in the same position as the most recent entrant, plus all of the changes which have happened since.