I'm not an expert on this, but I was looking into it a while back (for non-WAM purposes).
In your UMD profile, there are options for "Non-US citizen" (W-8BEN) and "Non-US business" (W-8BEN-E). Are you sure that you want to use the business option? I.e. do you have a limited company? If so, you'll need to contact the IRS and fill in form SS-4 so that you can get an employer identification number. The last time I checked, you need to phone them for this, i.e. it can't be done online.
NB In the UK, you can have a limited company with 1 director/shareholder. In the US, a corporation needs to have at least 2 people running it. So, if you're a "one man band" then you'll also need to fill in form 8832.
I'd also recommend setting up a Wise account, so that you can get paid in US dollars. (I.e. you get better exchange rates than a normal UK bank account.)