Hi,
I'm doing MFL (French and German) in the the North West. My French is to degree level and I did one year of German while at university and it's about at A-level in the English school system.
I would really suggest beefing up your Spanish so you could teach it, at the very least to the end of Key Stage 3 (year 9 aka 8th grade). Most schools here want to have teachers that can offer two languages because you're better value for money. Plus, Spanish, at least in my area, is becoming increasingly popular amongst pupils and teachers, so it would be a sound decision to offer it. Single language teachers, in my experience so far, seem to be a bit of a rarity.
For the application process, it can honestly be a real pita. There are a lot of hoops you will have to jump through. They might say you don't meet GCSE qualifications despite the fact that you have a high school diploma. If so, they will try to convince you to take equivalency tests. Make sure you have copies and your originals of your university and high school diplomas.
You will most likely have to be in the UK for the interview: bring those diplomas with you because they will want to see them as proof. Make sure you compile a list of your residences in the past 5 years so you can complete a CRB check form (criminal background check) either beforehand or on the day of the interview that the university requires.
Also, being American, they might require that you observe secondary MFL classes for a day or two to make sure that you really want to do the course. So, if you are in the UK for a few days before or after the interview, arrange to do those.
Definitely make an effort to research the UK school system before the interview too. You don't need to know everything, but having at least a good overview of knowledge of their system is a good idea. Key words to look up are the National Curriculum, Attainment Targets, Key Stages, Target Language, Four Skills, GCSE and A-level exams. A really excellent book that was on our bibliography which has many articles that give a good review of the history of the English educational system (and Scottish and Welsh systems iirc as well) is: . Try to find it at your school's library. I myself haven't bought it, I just checked it out from my local library in the UK, but it's a really fantastic overview.
If you have any other specific questions, feel free to post them here or pm me.