It is highly likely that the terms of a US/UK trade agreement would be weighted heavily in favour of the US. Even if there are no tariffs, there are non-tariff barriers which could be made to work in the US' favour.
Examples include the UK having to adopt US standards so that all US goods can be exported (chlorinated chicken and so on), and the UK having no quota for US exports but the US imposing very small quotas for UK goods.
In a negotiation between equal partners, the UK would insist that the US didn't subsidise (or that there were tariffs to compensate), but this won't be that kind of negotiation IMO.