You need to inform at the online school if the diploma guarantees accepting at English universities. Ask them if they provide online education to Spanish High School students as well. Look at percentages if available to know their success. Some online schools provide these information at their sites, as well as timetables/ schedules and their teaching methods.
I have the same plan as you, due to physical reasons. I would recommend you to orientate yourself on their sites, contact them and ask all the info you need. It's important for you to look at the laws in your country and in the UK, so you've prepared yourself. I personally think that any reason would be a good reason for homeschool education, but concentration is needed for a proper school atmosphere, so that's a very good reason. I wish you best of luck in your further search for information, and don't be affraid to ask too much, since any info is helpful ;)