Erm not quite.
The day 2 test kit reference needs to go on the passenger locator form (PLF), that you have to complete within 48 hours of your return to the UK.
You could have all the print outs, pictures, proof of purchase etc you want but unless you have the reference number to complete the form then you're liable for a fine upon arrival into the UK (whether they would or not remains to be seen)
And you can't bypass the part on the PLF where it asks for a reference, nor can you just put anything in that bit.
The reference should be five letters followed by 7 numbers; eg ABCDE1234567
(depending on who the kit was purchased with, you will likely need to register said kit with that company and also provide your return flight details to trigger the reference required)