How about at a later date?
The British win a resounding victory in the war of 1812, securing their interests in Canada, and imposing several draconian measures upon the USA in return for their nominal continued independence. The native American lands to the west effectively become a protectorate of the British Crown, at least for those who were allied to the British. The United States is weakened and cowed, for many it's seen as a return to colonial rule, in all but name, but if anything worse as the British battalions build in their defences, and control all the ports and major cities. Although the democratic system is still in motion, many feel that the Republican Party are nothing more than shills to the British Crown. The black loyalists and native Americans who fled America in the revolution now return, slavery is forcefully abolished, and there is growing resentment among the former wealthy plantation owners. Indeed almost all the citizens harbour some resentment, having lost a great number of friends and relatives in both wars. A guerrilla movement began to form, mainly composed of Irish immigrants of the Society of United Irishmen.
Meanwhile, after gaining Independence from Spain, the Mexican armies begin to encroach further and further into Native American lands, exacerbated by Comanche raids, and expansionist ambitions of the Mexicans. This didn't go unnoticed by the British, who feared that Mexico could annex almost the entire southwest, and become an unstoppable foe. The British troops began to congregate en masse in the ports and newly bult or restored forts throughout the colonies. The settlers began to fear a mass purge, but their fears were soon settled as the British battalions began to move out of the Colonies, and march west. As they went they fueled anti-Mexican hysteria among native American tribes, warning them that Mexico had ambitions to conquer and displace them. They managed to gather a large militia of armed natives, bolstered by warriors of Tecumseh's Confederacy who came down from the north.
As they reached the tribal lands on the border with Mexico, they began to set up camp, and in a show of strength made encampments from Texas right through to alta California. The Mexicans would have been wise to cut their losses, but they were bolstered by their recent independence, and were keen to make their own territorial claims. A huge force congregated in Chihuaha, and attacked a British outpost. It didn't take long for the British forces to recongregate, however, and they wiped out the Mexican Army wherever they found them.
Meanwhile, as the British troops moved out on their conquest, the natives of the USA took their chance. They set fires and attacked officials, all seen as part of the occupation. They hung those they believed to be traitors, including many recent immigrants who it was felt were put there by the British to dilute the nativist sentiment. In truth these immigrants were no different to their predecessors, many Irish and Scots who had no relation to the British Crown, as well as recently freed slaves and immigrants from British territories like India. In contrast, the Catholic immigrants from Ireland, and the Scots fleeing the Highland Clearances (perhaps still harbouring anti-British resentment after Culloden) were a huge party to the insurrection. In particular a mob formed which attacked an immigrant quarter, setting fire to houses killing 4 people, and hanging a further 3. These mobs would later fuel the rise of the notorious Clan na Celt, an extreme nativist group associated with lynchings, arson and bomb attacks for the next several decades.
In the meantime, the British didn't have the resources to deal with the insurrection. They had bitten off more than they could chew, with the Mexican rebels taking to the hills and fighting a guerrilla war for the next decade. The British decided it was best to gracefully withdraw from the former colonies, allowing safe passage to Canada, and keeping a close watch on the border. Apart from the short-lived American invasion of Canada, relations remained cool for the most part.
The British garrisoned troops throughout the country, and set up a puppet government. Surprisingly the natives of Mexico took to British rule, perhaps simply it was the lesser of two evils. Their territorial and native rights were generally respected, and the British were more secular, allowing them to continue their cultural and religious practices rather than imposing a strict catholic doctrine. With an influx of new immigrants from Britain and it's territories, a particularly strange dialect of 'Spanglish' energed. Although there was a certain amount of anglicisation, the introduction of beer and football for instance, most customs went largely unchanged. The British slowly withdrew, although Mexico remained in the Commonwealth.
And in the climax to this Brit-wank, the US becomes more and more dictatorial and corrupt, and soon American citizens start jumping the border looking for work..

(Too much?)
