ie. it takes it power frotm he PC then you are ok... .
Be careful with what you actually mean by this! If the monitor is supplied by the P.S.U. in the computer that may be true, but that's
not how the standard PC system works.
The IEC outlet on the back of a PC which you can use to power a monitor just provides whatever voltage you connect to the input. The switch (if fitted) which selects 120 or 240V adjusts only the power supply for the computer itself; it doesn't alter the output voltage from that IEC socket.
Many newer monitors use a type of power supply which will run on anything between about 90 and 250 volts with no adjustment, but check the label on the back for details.
How do I know if my HDTV will work in the UK.
It depends to what extent you want it to work. If you want to be able to hook up to a British TV antenna and receive signals directly off air, then it needs to support PAL system I. Few American-market receivers do, but check the technical specifications in the manual.