In reply to elliot.baker:
If it is an external wall, the plasterboard inner skin may be attached to it using thin wooden studs leaving a narrow (2 or 3cm) gap... or it may attached via 'dot and dab' where the plasterboard is stuck on to blockwork using big dollops of plaster. If internal new house it's almost certainly just a wooden frame made of 2 by 4" timber with plasterboard on each face.
You might possibly get away with drilling straight through the plasterboard into the block behind (if block). You can then (with a certain amount of care) put a suitable sized raw plug a turn or two onto the end of a long screw and use the screw to guide the rawplug through the hole in the plaster and into the blockwork. You can make the hole in the plaster a bit bigger to facilitate. Once the plug is in the blockwork try to remove the screw, leaving plug in place. Then mount the backplate of the TV bracket onto the plasterboard, putting fixing screws through the backplate and through the plasterboard and into the blockwork. A bit finniky but possible. If you carefully tighten all the screws you should get the backplate snug to plasterboard and well bedded into the plugs. The trick is to get it tight enough that screws are well bedded in the plugs and weight well distributed but not so tight that you crush the plasterboard. The risk if screws and plugs not long enough/not well enough bedded is that it sags as you are not really getting it as tight as if you were genuinely cranking fixes tight onto a hard surface. Only you can judge if you got enough fittings in well enough or not.
There may be suitable frame fixings of correct length to do the job easier than the above method but they will still rely on getting tensions right and not crushing the plasterboard.
Best fix creates mess and that involves cutting a section of board out, putting battons where you need them (if space allows) re-fitting board over top and skimming again.
I'd use a thin long drill bit and drill a few holes to figure out how deep the cavity is etc and if you find the bit goes in roughly 20mm and produces powder that's your plasterboard, then appears to hit a void that indicates cavity then grinds on stone that's your block. You can then try a masonry bit and test drill again into the block to make sure it's all as you expect and take it from there. If you only find a very small cavity or cavity varies from none to a few mm that probably indicates dot and dab and above method likely more successful as the gaps are smaller so scope for sagging/pull out less.
As somebody else suggested, don't drill vertically above or horizontally beside any power points/light fittings on that wall as cables should always run in straight lines up, down or sideways from any electrical point.
If making a mess putting batons in place you can extract extra value for the messy job by trying to thread power/aerial etc... any cables required into the wall if you don't want to see them/hide them in externally fixed trunking. Personally, I hate exposed cables....
Oh and for towel rails/mirrors etc unless very heavy, on plasterboard you would probably be OK with toggle fittings - these have spring loaded things that fold together, push through hole in plasterboard and then open wide on other side. Assorted sizes and strengths ranging from plastic types to springloaded metal. The only issue with these is you invariably end up trying to use them places where they just refuse top go in or open properly due to just being on edge of a joist you didn't know about, or fouling on insulation or one of the other hundred things that always go wrong with best laid plans.... Not uncommon to fit things using assorted fittings depending on what happens in each hole you drill, what you find!
Post edited at 13:53