A UK-based individual provides consultancy services to UK company

Hello, I am UK based individual person and will provide one-time consultancy services related to M&A project to UK registered company. 1. Should I register as self employed and pay income tax and NIC by myself? 2. What documents should I provide to UK company for the services provided (e.g. agreement, invoice and proof of work)? 3. What taxes, if any, does UK company to withhold, when making payment to me as an individual service provider? 4. If this is a one-off activity and not part of regular business-like activity, may HMRC treat it differently (not as self employed)? Many thanks for your reply.

Hi,
If your consultancy service annual gross turnover exceeds £1000, then yes, you will need to register for self assessment, as self employed Register as a sole trader.  
Please take a look at the Business records if you're self-employed for guidance on record keeping.  
How you invoice your client is up to you.  
If this is a one-off, you would still need to register as self employed.  
You would also need to confirm that the self employment has ceased.
Thank you.