This is on the origin Mexico store
NOTE: You will NOT be banned for this. Do not believe those that tell you this.
1. Make sure you have a debit/credit card saved as a payment option on your account. This is necessary
2. Install Hola extension to your browser, once installed, select Mexico. You will know it's worked as prices will be in US$ and it will be in Spanish obviously
3. Add the game to your cart, then click checkout, it will tell you to sign in if you haven't already.
4. It will bring you the payment confirm screen. I didn't have to disable Hola at this point and I just clicked pay and it added the game to my account, however if it gives you an error saying it can't authorise your payment then when you get to the payment confirm screen, disable Hola so you are back to your UK ip, it will refresh the page but keep your game in the cart with the US price, then you can pay, it will charge you in dollars.
Your bank may charge you for conversion fees, I'm with Lloyds and it charged me £1 but this is still cheaper than UK price