In the UK, I am lead to believe, the whole warranty cannot be invalidated by a change made to a part of the car, but any faults that develop as a result of a change can invalidate the cover for that item. i.e. changing a bulb for a HiD and the wiring melts (extreme example). Same with servicing - a manufacturer cannot insist that all servicing is done by the dealer network, but I think it would be an unwise choice not to do so, in order to avoid the argument. (see below)
Changing a like-for-like bulb (55w standard for a 55w Osram) would, I am sure, not invalidate the warranty as the manual does not mention name of bulbs, just the type. Defence would be - the bulb blew so I changed both bulbs as the other one may be due to fail as well. Changing to an off-road 100w and melting the fittings would invalidate the warranty!
There was a test case a few years ago on a P.C. - there were stickers on the case saying that opening the case would invalidate the warranty. When a problem developed, the stickers were seen to have been damaged (stuck over a join in the case) and the warranty cover was declined. A judge ruled that a P.C. was inherently updatable as it had spare slots and a number of add-ins were available on the market, so "sealing" a case was unreasonable.
Jeff
Q&A - Automobile Association (UK)
Do I have to get my car serviced by a main dealer?
You are not obliged (since October 2003) to get the car serviced by a franchise dealer during the warranty period.
You must though get it serviced according to the manufacturer's recommended schedule and criteria using only manufacturer approved parts.
The legislation only applies to the part of the warranty offered by the manufacturer, i.e. insurance-backed extended warranties offered by dealers are not covered and may stipulate different conditions.
If you do get the car serviced outside the dealer network you must make sure it's done to the maker's recommended schedule and criteria using approved parts. You must also keep records so you can demonstrate to the manufacturer that servicing was undertaken to their requirements.
You should still get warranty work undertaken by the franchised dealer as it is easier for them to deal directly with the manufacturer.