Your poster needs a size too.

<video width="100%" height="100%" controls >
<source src="https://initiostar.co.uk/assets/F28 Movie v8 1000kbps.mp4" type="video/mp4" >
<poster="https://iflewformma.com/xara//index_htm_files/f28poster.jpg">
<width="480" height="260">
</video>

Size to suit your poster.