The USA and other countries we or the EU trade with are subject to separate negotiated agreements.
The current UK-EU Trade and Cooperation Agreement (TCA) is dependent on the UKs continued membership of the ECHR. Leaving the ECHR is likely to be considered a breach of an essential element of the TCA (the Parties reaffirm their respect for the Universal Declaration of Human Rights and the international human rights treaties to which they are parties (Article 763)) which will enable the EU to terminate or suspend the operation of this Agreement in whole or in part (Article 772).
While unlikely to lead to a complete suspension of trade, it will mean that further renegotiation of the deal to remove the onerous requirements on paper work, it would end any prospect of better deals for animal and plant trade. The price of food and other goods would rise as a consequence and the trade already lost by thousands of small to medium suppliers will never be recovered.
The UK has already lost around 15% in export trade with the EU at great cost to the economy and indeed us its citizens due to Brexit and specifically the fact that despite the claims of those on the Vote leave side, that the EU would be eager to trade on the same terms, funnily enough they weren't and didn't.
The prospect of more friction on trade and no prospect of reducing the existing costs and friction imposed under existing deals with the EU, would inevitably result in prices rising and trade declining.
In addition of course, the Belfast/Good Friday Agreement (GFA), which has helped to secure peace in Northern Ireland, is underpinned by UK membership of the ECHR. Even that know purveyor of fantasy policies, Farage admits that the GFA would have to be renegotiated no easy feat when the initial agreement required referendums on both sides of the Irish border.
And all for what? Leave the ECHR and kiss goodbye to returning failed asylum seekers to the EU, they won't accept them.
Amending it is a possibility, but complicated, but in any case, contrary to what certain media and other commentators report, its application is only affecting a very few cases where the UK wishes to deport people. So the benefits will be negligible, set against the disbenefit.

