Don't need too! Let's put it another way. If we sign a trade agreement with another Country. We don't need to offer the same terms and conditions to any other WTO members.
Just quoting what many others on this blog have said. Yet the same people seem to think we don't have the right to sign trade agreements. If we do, then we will be in conflict with the WTO.
I'd suggest you really should go read up, as you aren't getting the detail correct.
To try make it simple, if you trade with a country on WTO rules the conditions/tariffs applied must be identical to every other country you trade with on WTO rules. So if we traded with the EU on WTO rules and dropped all tariffs we would have to drop all tariffs to every other country we trade with on WTO rules.
If you sign a trade agreement with a country or bloc you are trading on the agreement rules with that country or bloc, not WTO rules.