The most important thing about flashing is to make sure that the n900 is fully charged and you have a little time to reconfigure it afterwards. Do a backup before you start and it's a surprisingly easy process. I can't imagine any warranty problems - I would be very shocked if I and all the others who have made the change were to be told that our kit was no longer guaranteed. The good thing about a community like this is that we help Nokia develop their product but we can probably make a significant amount of noise if they start doing something daft like saying that changing the locality of your firmware voids your warranty. Flashing is not difficult or dangerous - search, read the guides, follow the preparation isntructions and you'll be fine. That said, this version of the firmware is not a lifechanger and it's possible that the UK will get the next big change at the same time as everyone else with this one bundled in, so there is really no need to rush into it.