It would be easy to look up online the make /model and 2nd hand value. Your use of GIVE not BUY is key here.
If you did that it would clarify in your own mind how much of a favour it was.
You could also factor in the time/effort cost of finding a charity shop that would take it.. Or how much your council charges to collect large electronic items?
How far off the value would the £20 be then?
Often people give things to relatives as its an easy way to dispose of good condition but unwanted items... its easier than bothering to go through the motions of advertising and selling. Some relatives might think its sort of saying to them, I'm better off than you. Only you know what they are like.
The relatives also knew that you could afford to upgrade and didn't want the existing TV anymore.
"Give what you can afford" is a difficult thing to decide. In "market" terms, they weren't shopping for a TV, but you were offering one for them to take it off your hands as you didn't want to dispose of it yourself.. so they thought OK then..what's that worth to us. What can I afford to pay for something I wasn't particularly looking for? Not to forget the concept of mates rates either.
I agree they were rude not to even say thank you.
But They did give you £20, which you feel is insultingly low.. They must have said some words as they did this and been friendly? I take it they weren't actually uncivil to you when you gave it to them?
But expecting flowers when what you asked for was an unspecified amount of money and for them to take the TV off your hands so you could get rid of it is a bit of a muddled concept.
I think in your positioni I'd probably resolve not to give them things again if I wanted money for them or at least say its worth this, would you like to buy it... but at the end of the day, you've got rid of the old telly and have a lovely new one.. I'd let it go. Its not an important thing to hold on to.