At minimum (so could be higher,) with any disputes capable of being enforced from any member country. If someone wants to sue you from Hungary, they can do so and can expect an equivalent result as if they sued you in UK, Portugal, or any other Single Market (SM) member country. Though the rules are set by an "external" power, at least the UK had a vote and some representation on what the rules and trading policy is.

Now? Market size of the EU means that it's impossible to ignore the rules of your largest and most importantly nearest trade partner. So practically speaking UK businesses are effectively tied to these rules anyways, now without any votes or representation.

That's just how it is when trading. End of the day: You must be compliant with the importer's standards. Inside the Single Market this would be baked into your rules, so there's little need to have documents checked at the border between members. With EU membership you'd even have a vote on what those rules are. Outside of it like any other 3rd country (US, China, Australia, aka non SM member) you must prove it via documentation and provide it at the border into the SM, no matter how closely your country's laws align with said rules.