What Is The Best Way For It Contractors To Pay Less Tax Legally In The Uk, Eg Umbrella Company Etc?

You can set yourself up as a Ltd company, and pay yourself a base minimum salary to get NI stamp, etc, and pay the rest in dividends, which are taxed less than regular PAYE salary. You can do this if your contract of employment is a “contract for services” and so falls outside the IR35 legislation, as opposed to a “contract of service” which is like regular employee type agreement.
Google on IR35 for lots of info, but there’s no clear cut definition of what you have to do. You’ll need an accountant that is familiar with IT contracting and IR35 issues.