If it’s the first time you’ve done this TFL will probably give you a straight refund if you talk to them, but if it happens again they may ask for proof of the e-ticket. That’s what happened when I went to football in the midlands and absent mindedly used my card instead of my paper ticket to exit Euston mainline when I got back.
Also, just in case you weren’t aware, if you can board at Luton Airport Parkway rather than Luton, contactless (but not Oyster) would be valid for the entire journey without the need for a separate ticket to St Pancras.