Those freepost mailings can take ages, they're not high priority. Lower than 2nd class in my experience.
I used to trade in my old phones with Envirofone, and I'd be fretting for a week before they acknowledged receipt.
Also there seem to be postal delays of late, I've been waiting 4 days for something posted to me first class - still not arrived. My wife's been expecting mail too. May be some sort of strike action at Royal Mail.
Obviously you should have sent the package recorded, or better still, special delivery (as you would have been insured if it went missing). Standard post with proof of posting (which you didn't get) is only about £30 of cover I think.
Don't panic yet though, give it a few more days
SV