The real problem with a larger Mexican cession is that of all those Mexican territories, Yucatan is the only territory that can be guaranteed to allow slavery. Maybe the crypto-Jews of Rio Grande see the political necessity of permitting slavery on their side of the Rio Grande. Getting legal slavery into Chihuahua, Durango,Zacatecas, San Luis Potosi, Sonora, Sinaloa and Baja California is going to be a big issue, since the people there are used to not having it there.
What is this supposed to mean?
I find that an interesting possibility. I wonder if the Brits would build an early Nicaraguan canal?And the extension of slavery to Yucatan is going to be an emotional issue for the British public simply because Yucatan is formerly free soil to which slavery has been extended, it borders on Belize and there's an insurgency of Indians fighting to avoid being enslaved to which the British can contribute arms. Ordinarily the British might not want to offend the United States, but the extension of slavery to formerly free soil may trump that consideration and put the US in the same pariah category, as far as the British public is concerned as South Africa under apartheid was IOTL. We could now easily see Great Britain, for example, taking over Guatemala, Honduras, El Salvador, Nicaragua and Costa Rica, oestensibly to keep them out of American Slave Power clutches. The ripples of the US avoiding the Crisis of 1850 at Mexico's further expense (if that crisis is avoided considering sentiments about slavery in some of the parts of Mexico the US is annexing) may spread far beyond Mexico.