...a free trade agreement with the EU and most of the western world à la the US. They still have plenty of leverage as a consumer of European products (esp. cars) and a supplier to European demand.

Their position attenuates slightly if Scotland secedes, but not to a large enough extent that they are unable to negotiate said agreements.

One Brexit side effect may be better trade relations with the US, to American benefit. Europe has now devolved into two behemoth economies from a previous singularity.