Why can’t I use England/Wales/Scotland flags on excel for iPad?
I’ve tried deleting and re installing the app and doing all apple updates
Do you mean the Emoji keyboard symbols?
If so its because Excel does not yet support this character in the font and is showing for Scotland a black flag followed by gbsct<cancel tag> as a 'subdivision' of GB.
The only hope is to try changing the font of the cell before typing in and seeing if that works long term:
Excel Home Tab
Click Font (defaults to Calibri 11 on my copy)
Scroll down to iOS fonts
Select 'Apple Color Emoji'
Type in the emoji
At this time it shows up correctly in the cell when entering the data or editing a cell but not when you complete the edit or in the text box at the top!
I'm not sure why Microsoft has not fixed this as you would have typed it in so its not impacted by https://9to5mac.com/2018/02/02/apple-rejecting-apps-with-emoji/ and other ones work.
You may get some luck on the web site Customer Feedback for Microsoft Office 365 and adding your vote to the requests.