I teach science subjects at A-level which have essays with my exam board. The essays are "ladder marked" which means you put the essay into a band based on certain criteria, and then you look further at the content of the essay to determine whether you give it, say 25 marks (top of the band) or 20 marks (bottom of the band).
There are no separate marks for "quality of written communication".
For the top band for biology, the criteria is:
"Response shows holistic approach to the question with a
fully integrated answer which makes clear links between
several different topics and the theme of the question.
Biology is detailed and comprehensive A-level content,
uses appropriate terminology, and is very well written and
always clearly explained.
No significant errors or irrelevant material.
For top marks in the band, the answer shows evidence of
reading beyond specification requirements."
The main focus is on breadth and the content, rather than the style of writing. You can have less than perfect sentence structure in places and get into a high band. You can make spelling errors- as long as these don't cause confusion between scientific terms, these are usually ignored.
So, for example, if a student writes "meitosis" that would be given credit as it could be mitosis or meiosis, but if they wrote meiosiz, they would probably be given credit as it's clear what they mean.
From my experience looking at papers we have been sent back from the exam board, generally detailed discussion of a breadth of a-level biology topics gets the higher banding- a really well written essay with poor or narrow subject content wouldn't get top marks.
I've never personally marked a student 25/25 but I wouldn't not do so on principle and I wouldn't not do so based solely on spelling and grammar errors. According to the examiner's reports, every year about 0.5% of students are marked at 25/25 on the essay by the exam board- so there must be a number of teachers across the country with these students in their classes.