I want to unlock my iPhone carrier lock

You are not talking to Apple representatives here. Regardless, Apple cannot unlock the carrier lock on this phone. You need to contact the UK carrier to have them unlock it for you so that you can use it with another cellular carrier.

Ref: How to unlock your iPhone for use with a different carrier - Apple Support

Nobody on these boards can, and nobody at Apple will. In addition, not all carriers offer unlocking.

Click here and find the carrier it's locked to.

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You have to contact your carrier and follow their procedure.