It's generally accepted that using grammar checkers to improve your writing is not plagiarism. Many professionals and students utilize these tools, which are intended to help with language and writing improvements, to polish their work. It is important, although, that you comprehend the particular guidelines set forth by UCAS.
To be sure, carefully review the guidelines provided UCAS. If you have concerns, you may want to check with your advisor or the relevant personnel to ensure that using grammar-checking tools aligns with their policies.