Active Directory Name Space planning

Can any on give me advice on the best practice for
DNS naming in a new Active directory.

Should it match the existing company domain
name? Reasons for and against?

Example scenario

Existing NT domain is named “ABCUK”
Single site, singledomain
Web server is named www.abclaw.co.uk
Email addresses are @abclaw.co.uk
Using a 192.168.1.0 subnet with NAT to
the internet.

Should the active directory DNS be

abcuk.co.uk
abclaw.co.uk
something else?

Thankyou for time