Sending your old passport back when you renew your passport

Dash22

I just used an ordinary 1st class stamp ( not large letter ) on an old padded envelope, and put it in the pillar box in my local Tesco's  and got confirmation of its arrival at the Passport Office three days later - easy peasy - no need for tracked or any other expensive means of sending.

Annemos

I prefer to take their advice and send it Royal Mail Special Delivery. It did not cost much. 

basill

The passport renewal depends on receipt of the old passport.   If time is an issue use the very reliable tracked special delivery.
Its fine to use ordinary post if you are in no hurry and have time to deal with a delayed or lost document.