citroen Every country theoretically has "access" to export to every other country or trade bloc.
However, there are are normally quotas, fees, certification procedures, tariffs and many non-tariff barriers that prevent anything like as easy or cheap access that members of a trade bloc have.
e.g. looks like components criss-crossing EU borders umpteen times before reaching an assembly line would not be practical without belonging to the single market and the customs union, or an EEA / EFTAA type Brexit
Turkey belongs to the Custom Union and has theoetical "access", but lorries at its borders wait typically 15 hours, sometimes 36 hours, in long queues to enter the EU.
That would hammer UK manufacturing exports, or component imports.
Canada has CETA, which Brexiters think could be a template deal. However, they don't export services to the EU and much of their exports are raw materials.
Their meat industry has reported that despite CETA supposedly allowing it, they don't have access in practice because of EU regulations conflicting with those they must fulfill for the US market, with both the US and EU requiring their rules be universal, to avoid accidental "contamination" anywhere in the chain.
To continue the highly lucrative City financial services, the UK would need Financial Passporting from the EU to do such business involving EU countries.
The EU probably can't immediately replicate all London services, but they will phase out UK passporting as they develop their own capabilities.