Twin wall flue distance below ceiling
In BS EN 15287-1
Changing from single skin to twin wall flue below a ceiling
When you change over from single skin flue pipe to twin wall insulated chimney/flue below a ceiling this should be done a minimum of 425mm below the ceiling according to BS EN 15287-1. There is also some guidance in Document J about the distance which single skin flue pipe should be away from combustibles.
Additionally the join should be in the same room as the stove - so you should not run the single skin flue pipe through the ceiling. Running a single skin flue pipe through a ceiling is a pretty bad idea because the ceiling is likely to be flammable and because it is not a good idea to have a very long run of single skin pipe anyway.
Practically speaking we recommend just using twin wall flue/chimney straight off the stove as it will look a lot cleaner and the chimney function will also be improved through being better insulated. BS EN 15287-1 also confirms that you can connect the twin wall directly to the stove without any single skin flue pipe.