Upvote:1
Everybody needs to be able to enter the US to transit there. This means that as a UK citizen, you generally need an ESTA for your transit in Dallas. If you would fly to Mexico directly, you would not need an ESTA but could still use the Visa Waiver Program (i.e. you do not necessarily need a visa).
But because travelling to Mexico does not extend or reset the 90-day period of entry, the clock would start when you transit in Dallas and the whole Mexico-US part of the trip must be shorter than that. See CBP FAQ about the VWP for more on this. If you are staying longer than that in total, a visa would then be required for your stay in the US.
But all that is mostly relevant if you only have your UK passport with you and can't prove that you are also a Canadian citizen. There are other rules for Canadian citizens (and in some arcane details, for Canadian residents but that's probably not relevant here). If you have a Canadian passport or, if entering by land, another document compliant with the Western Travel Hemisphere Initiative, it would seem easier to use that.