You can go here to get an estimate for it directly from Apple, assuming they are still taking in 4th Gen iPads: Apple Trade In - Apple (UK)
If its not an option in the Estimate calculator there, it means Apple is not taking them for trade in anymore, and the value would be whatever someone is willing to pay for it. Ebay shows them to be anywhere from 50 to 200 pounds depending on storage capacity. https://www.ebay.com/b/Apple-iPad-4th-Generation-Tablets-eReaders/171485/bn_320814?rt=nc&LH_PrefLoc=5
Its original retail price would depend on its storage capacity and whether or not its the cellular model.
See here: https://igotoffer.com/apple/ipad-4
Also, you should wipe the ipad clean before selling it, or trade in it in. It should not have any apps you downloaded or your Apple ID. Follow the steps here to get it ready for sale or trade in: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support