I have had a number if cars remapped by Superchips in Buckingham. Never had an issue with warranty. The installer always said that because you are not replacing a physical chip; more recoding what is already there, it can't be "seen". You should always tell your insurer though.
The first car I had done was a 6 month old A4, and it was actually the dealer that introduced me to the concept. It was a 104ps but I wanted the 130ps, but the dealer did not have one and i was looking for an ex-demo. So to tempt me into the sale they said I could get it remapped, and they even took it there for me before I picked it up. So they clearly weren't too fussed about it.
Equally, the remappers say that it causes no issues for the vehicle as long as it remains within certain tolerance's, and the UK models are de-tuned anyway.