iPhone is locked to owner

See -> How to remove Activation Lock -> How to remove Activation Lock – Apple Support (UK)

• If you need help removing Activation Lock and have proof of purchase documentation, you can start an Activation Lock support request.

You can also contact Apple support for help, -> Get Support (apple.com)

You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.

If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.

Once Apple receive your proof of ownership, Apple will further review your request and provide an update.

Read this before you buy a 2nd hand iPhone.

If your phone is Activation Locked only the person who locked it can unlock this theft deterrent feature. See→Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Well, of course it doesn’t work unless the activation lock was enabled by you using your Apple ID and password. If it wasn’t you might as well throw away the phone, or better yet recycle it, because you will never be able to use it.