Does my GCSE grade affect my entry?

Honestly GCSE grades can work both ways, some unis have a scoring system whereas others just want a minimum which is usually a minimum amount of A/A*'s and a A/B in maths and english. Having said that you can apply to med school with fairly average GCSE grades as long as you apply strategically, check the minimum requirements of unis before you apply. A lot look at UKCAT/BMAT and A Levels, so also focus on getting a good UKCAT/BMAT score.

Ohh okay i was actually looking at BMAT tests as i was looking at entry requirements or russell group universities and they only really look at maths and english, i just needed some reassurance and thanks for your help. So just to clarify it really matters about UKCAT/BMAT scores and of course your A-Level Results in contrast to a biology GCSE results?

Some medschools are more interested in your overall GCSE profile than others.

If your GCSEs are slightly weaker just avoid those medschools.

See the full details here (but double-check individual medschools own webpages because they can change...)
https://www.medschools.ac.uk/media/2357/msc-entry-requirements-for-uk-medical-schools.pdf

Nah GCSE mock results aren't bad acc they came out with a couple 9s 7s and a 6 so i just wanted to clarify if they really care about biology GCSE or that my other GCSE grades will make up for it but on the attachments the universities that i want to apply for only care about english and maths and BMAT

Which universities are you thinking of?

UCL, Imperial College and Kings College

Ok, and what is the attitude of those medschools to GCSEs?

Also, does your planned sixth form allow you to do an A-level in a subject you got a 6 in? (And just to add, this is why it's often a bad idea to sit exams early...)

yeah majority of sixth forms have the minimum entry requirement of a 6 some have 7s but I've got in and i was forced by my school it wasn't optional if you were in set 1 in my school and they only told us 5 months before we actually were sitting the exam.
The attitudes are only towards English and Maths with a minimum of a 6, which is doable, but my issue isn't that my issue is if they look at that grade will they automatically defer my entry of will they mainly look at academic test e.g UKCAT/BMAT and obvs A-levels, personal statement?

Defer? You mean decline...

No, unless they say otherwise your A-levels, etc are more important.

Also, you should ask your referee to mention the circumstances behind you taking it early. Sounds like a very stupid rule...