Hi Nico, I'm doing exactly this. I'm 2 months into my METV, so watch this space!
Only thing to note is that to get an METV (in the UK anyway) you need a return ticket within the 6 months. We picked the cheapest one way ticket to a neighbouring country exiting just before the 6 months (remember it's from ISSUE not when you enter the kingdom). Whether I choose to use it or not...😉
I'll update the group through my progress as it might just be useful!
All you need is a copy of the SA100 that you submitted (my accountant does mine but sends me a copy each year to sign saying I'm happy for it to be submitted).
I've recently got an METV in the UK as a self employed person. I used a copy of my last tax return. I also have a LTD company so was able to supply the company registration doc (certificate of incorporation). If you're not a LTD co you won't need this. I got mine from Hull. They were excellent. I emailed them all my docs beforehand, they checked them and confirmed they had everything, and then I just went up and paid for the visa. Was only in there for 20 mins.