FA kit regulations are here
https://www.thefa.com/-/media/files...-updated/kit-and-advertising-regulations.ashx
I would be surprised if the Palestinian flag on the sleeves was rejected. The FA regs say 'A single image of the national flag of the country to which the Club is affiliated may appear as one single area on each sleeve of the playing shirt provided that the rules of the Affiliated Associations and the Competition so permit and that it does not exceed an area of 25 square centimetres.' The UK does not currently recognise Palestine as a state but has commitments to do so depending on the circumstances, and the UN describes Palestine as a state ('non-member observer state'). Also highly significant is that Palestine is a member of FIFA.
However the kit badge in the photo superimposes a territorial extent in front of the flag and I would expect that to be rejected as being a political image. I would also expect the slogan on the back of the kit to be rejected.
The shirt might also be considered 'very dark' in colour and be rejected as potentially clashing with referees' black kits.