I always associate picture rails with pre-war houses, and especially Victorian era homes. They fell out of favor in later years, except for specific builds or remodels which were trying to emulate the earlier style. I'm speaking of my experiences within England -- Regional variations may be a possibility.
Don't forget to use one of those wiring detectors to ensure there's no live electric cabling where you'll be drilling...
Just to expand upon that, for some years now it's been part of the British electrical code that cables located less than 2 inches below the finished wall surface should be run in so-called "safe zones." These are the areas of wall which extend vertically above and below, and horizontally outward from any switch, outlet, wall light, or other electrical fitting. There are further 6-inch wide "safe" zones designated horizontally along the top edge of the wall, horizontally along the bottom, and vertically at each corner.
So in theory, anything wired in the last 25 years or so to code should not have any cables located less than 2 inches deep at any other point on the wall.
In practice, of course, you'll find plenty of older places wired before the "safe zone" idea was adopted, plus plenty of rewires, alterations, and extensions which have been done any old way. It's not at all unusual to find cables buried barely an eighth of an inch below the plaster surface and running
diagonally across the wall! Good luck trying to anticipate such cable runs!