Best courier service to return IPhone

Lack of acceptable/reasonable service?

Best to invoke "right to reject" rather than your indicated intention of "right to cancel",
where r2r legislation means simply (quote);

the consumer has a duty to make the goods available for collection by the trader

so no obligation for you to identify or pay for the return,
assuming the supplier actually wants it back and can be bothered to collect.

So "just" inform the trader of order rejection rather than cancellation, and wait for the supplier to collect, or accept the supplier's return label if you are flexible enough to take the rejected item to the supplier's preferred courier's premises.
Do not part with the goods until the supplier's agent provides you with a receipt.

Evri are supposed to be good

Only use Royal Mail special delivery
Do not use any other courier, too risky and long drawn out compo process of it goes wrong.

Royal Mail Special Delivery plus you pay for additional insurance to cover the cost off the phone.

Edit: (deleted as rubbish)

(edited)