Are you seriously blaming a pot plant for the downfall of BioSoc? We stopped for exams anyway. And we think the Venus Flytrap is locked in Lab 42.
Maybe...
Oh wow, well done on your results!
I applied to Woodhouse for sixth form (as well as internally to Latymer), so I went to the Woodhouse induction day and had sample lessons etc, so I'm going to assume Woodhouse and Mossbourne are similar. It is true that Latymer has a "head start", if you like. Because of that, Latymer will expect you to do a lot of the 'extra' work yourself - Latymer will have more experience with students of your academic aptitude, and so they'll know exactly what you need to do to get an A*, but Mossbourne would probably be hammering the syllabus into you throughout the year, whereas if you go to Latymer it's largely up to you to push yourself to revise it in your free time. If you're struggling, you absolutely can go to see your teacher at lunch (or in one of your free periods, if they're not teaching) but I'd assume that at Mossbourne would have a formal support system in place, whereas beyond year 11 at Latymer, academic support is very informal.
It is also true that at Mossbourne you'd probably be one of the academically brightest in your year, whereas at Latymer, although from your grades you'd probably be somewhere vaguely near the top in terms of academics, around 1/10 of people in my year got all A*s at GCSE. So if you got an A or A* at Mossbourne, you'd be praised etc, whereas at Latymer the first question would be "Well, how high was your UMS?"
It all depends how you respond to praise, and also how confident you are in yourself - a lot of external applicants have been top of their class for 5 years, and find that suddenly being in an environment where everyone is as academically intelligent as you really impacts their confidence, but then some people thrive on the competition, and do really well at Latymer.
You also make a good point that, at least for uni, getting 4As from Mossbourne would look better than 4As from Latymer. That's not to say that getting 4As from Latymer is a bad thing, but it's somewhat assumed that a Latymer student will do well in exams. But you should try to pick the school that's right for you - not the one that will look better on your UCAS. If you're not going to a school that you like, you're probably not going to succeed as much as you would otherwise.