but I finally found it.

The UK is not and never will be in the eurozone, so with regards to finance they will retain their own policies. But to step outside of the single market would be a blow of massive proportions to the British economy, and British business is very much aware of that. And while it's not impossible to be in the single market while not being in the EU itself - Norway and Switzerland are in that position - such a thing has never been tried with an economy as big as the UK's, and it inevitably entails the obvious consequence of having to swallow some things which you don't have any say in. If you're a EU member state, there will also be regulations that are passed that you can't block on your own; but as a non-EU single market member, there are regulations that are passed that you can not only not block, but that you don't get the slightest say in. Albeit obviously in rather fewer domains.

Point is, it's obvious that Britain is far better off with something close to its current position of EU member with lots of special exceptions and opt-outs, than it would be as a non-EU member. And as the Economist argues, despite the resentment in many EU countries at the notion of the already privileged UK trying to negotiate yet more special exceptions and privileges, there may be political reasons why they could still have some success.