We have just finished our self build. It certainly tested our marriage and I won't be doing a 2nd one!
Where will you live during the build?
Agree with making sure you keep and organise all receipts for the VAT return, we found it a pretty straightforward process but that was because DH was meticulous at keeping all the paperwork.
Plan your kitchen at the earliest stage possible, even before planning if possible, and I mean down to where all the cupboards etc will be. So many people get to the kitchen design stage and wish they had made minor tweaks to their plans to allow for a walk in pantry or to move a window etc.
Expect it to take twice as long and cost twice as much as you originally planned.
I certainly got decision fatigue. I found the kitchen quite enjoyable to plan, but other areas like bathrooms more difficult.
I found loads of local self builds on Instagram, that's where I got most of my inspiration and recommendations for different companies etc.
I'm so glad we're in the house now, but if I'm completely honest, there's part of me that wishes we had just bought a house and done some decorating/put in a new kitchen etc. it would have been considerably less stressful and would not have taken over our lives like the self build did.