I don’t have the option to format the date correctly in the spreadsheet.
It should be dd/mm/yy but that option strangely does not exist.
I change the formatting to Numbers General and it lets me enter it correctly only if the day is over 13 (say, 22/6/19). Otherwise it changes back to date and displays incorrectly.
Is there no way to set this as user defined so that i can enter the date the correct way?
For whatever reason you want dd/mm/yy, please consider this is an ambigous format. In that sense, I wouldn’t call the requirement a correctly date format 
- have a look at ISO
My reason for wanting to do it that way is that is how I was taught more than 40 years ago and I have been doing it ever since.
Sorry, I did not and do not want to offend you, but your reason doesn’t change the fact that the format is ambigous and I have absolutely no desire to convince you to not use the format you want to use. My comment was just meant as a hint.
No worries. No offense taken. Just after doing it that way for four decades, that is what I am accustomed to.This is for my own files, so it’s not a big deal. I understand that for other applications it would be “backwards”.
Normally, formats for a date are determined by setting the language in LibreOffice.
At “Date acceptance patterns:” you can see what entries are available. You can add your own.
You can format cells for a date by going to Menu Format>Cells. There you select date. You can also specify a custom user-defined format at the bottom of the dialog box.
If my answer has solved your problem, please click on the checkmark ✓ in the circle 
to the left of the answer and click on the arrow ^ for upvote. This will tell the community that the question has been answered correctly.
With me Windows 10 Home; Version 1903; 64-Bit | LibreOffice, Version: 6.2.5.2 (x64).
Just to emphasize that even if the locale is not English-UK (or any other locale where the default date format is not DD/MM/YY) one can still apply such display format by manually editing and applying the format code DD/MM/YY
What cell input is accepted as valid date content depends on the Date acceptance patterns highlighted in the answer above. The initial default date acceptance patterns depend on the default locale.
