Sorry I did not look up the model as it is missing the P I see now, but its all the same for all the android devices between tablets and phones and I nearly always have phones here.
What we do is try to flash the stock image onto the Tablet via the computer.
To do this, you have to have a windows computer.
Then do the following please.
Download the firmware you want to downgrade to at the following sitehttp://www.sammobile.com/firmwares/
You enter in the model GT-P7510 as you forgot the P
Download the UK one 3rd from bottom
Once the download is complete, extract the firmware file to a place you will remember, like a folder on your desktop.
Download Odin. Odin is a simple utility that is used to flash firmware to Samsung devices. Here is a link to it Click me to open
Make sure your Tablet is off and disconnected from your computer.
Click the PDA box and browse for the extracted firmware file.
Then on the Tablet, Press the power button and volume down simultaneously
After a few seconds you will see a box with an android inside saying “downloading”.
To continue you must press the volume up button. Then a screen that gives us the option to continue or cancel.
Press volume up to continue and you will see a new icon that says Downloading.
Now connect your Tablet to your computer via USB.
In Odin, look to the top left and wait until you see in the leftmost box "COM" followed by a colon and a number, such as "COM:4". The number doesn't matter. Once you see that, you can proceed.
In the Message box in Odin, you should see several lines of text.
Press Start in Odin.
Odin will begin processing, and on your screen you will see "Downloading" with a status bar underneath.
When Odin is successfully finished, you will see in the Message box "All threads completed".
Your Tablet should begin rebooting. You can disconnect from the computer.
If your Tablet reboots successfully you're done and it was able to be flashed
Let me know the results please