Trade deals also include an agreement to collaborate on standards, etc. This is why a US deal is so controversial, because they want to sell us food that is not made to our high standards. This leaves three options:
1. The US agrees to match our welfare standards in food production, and therefore agrees to change their law in order to sell to our market -- and, moreover, to preserve that change in future;
2. The UK agrees to relax its standards, and to preserve this change in future;
3. There is no agreement, and both sides keep their laws exactly how they want.
In scenarios (1) and (2), one nation or the other (or more likely both) agree in effect to change their laws to satisfy the demands of the other. That has a material impact at least on absolute sovereignty, in the term it's usually used when talking about our relationship with the EU: we agree to change our laws in order to relate with other nations.
The thing is, though: so what? The principle of absolute sovereignty appears to mean that we should never do this, but we are not being forced into anything. In any case, the nature of a trade deal is that we also end up exerting influence on the laws of other nations too. Does this destroy their sovereignty? No. They agreed to it.