The 10% relates to possible tariffs if we leave without a deal. I really can’t see that happening, I reckon article 50 will either be extended another six months or a deal will pass at the 11th hour. Of course tariffs are only one factor. The £/€ exchange rate has devalued by around 20% since 2016 hence why a £28k GTI is now a £34k GTI. I also changed my car for March rather than September when it was due for the same reasons - plus with the mk8 coming along I didn’t want to be left with mk 7.5s in short supply or mk8’s with no early discounts (or not even available until 2020).
Back on the subject of 10% tariffs, the UK reserves the right not to apply them. There’s a risk the EU would not reciprocate on UK car exports (Honda/Nissan/JLR) but it’s an option nonetheless.
My personal view is that tariffs will be mutually damaging and government/business will find a way around it. I don’t buy all the scare stories. I also don’t buy the utopian Brexit dream. The reality lies somewhere in between.