On a technical note, I'd reccomend checking out installation guides that focus on thermal bridges and airtightness - you can make a huge difference to your buildings performance quite eaisly at this stage. Somethink like this: https://www.ecologicalbuildingsystems.com/post/airtightness-window-junctions-sealing-tape-or-liquid-membrane

essentially, but check with them about the specifics. They may charge more.

Yes, you can fit them yourself, but you will need to have the work insepcted by the building inspector (council or approved third party). Contact them and ask them what evidence they need and what visits they will need to make so that they can pass the work as satisfactory.