How to explain bad GCSEs in personal statement?

No, your personal statement is not the place to explain past academic performance. You should use that space to demonstrate your passion for biology by talking through any super-curriculars you've done, not making excuses.

Ok thanks. In terms of extra cirricular activities I can think of, do you think things like visiting science museums or doing my own research into various fitness topics (i.e nutrition and exercise) would be worth talking about?

Yeah but be specific, what interested you or caught your eye during your visit and what did you do to find out more about what you saw, and why was it interesting? Give 3 or 4 solid examples like this and your personal statement will be good, they'll see that you have a passion for biology, you won't even need to say it.

Yeah that's a great idea I will be sure to go in depth thanks. Last question, obviously I will have no choice but to mention the business BTEC that I spent 2 years studying, I can't just ignore those 2 years of my life. Do you think I should mention something about how I initially thought this was the right choice for me, but during the BTEC my passion for biology grew further and by the time I realised this it was too late to give up on the BTEC? I mean I'm just trying to figure out how I will explain that part. I guess I can talk about the skills the business BTEC taught me such as research and time management skills, but talk about how biology is still my passion and I came to fully realise this on my gap year which is what pushed me to do an access course in science instead of going to uni for business (this is the truth)

Ignore your BTEC entirely, it simply isn't relevant to your application and shouldn't be talked about. Your personal statement is an opportunity to display the fact that you are willing to spend three years of your life immersing yourself in one subject, you do not have the space to talk about why you decided to take a BTEC in Business and, frankly, the universities you're applying to don't care.