Macbook pro with retina display two weeks ago from Cardiff, UK apple store, once I put it into the charger it's getting extremely hot !! note that I only charged it twice since I bought it !!! what should I do ? should it be replaced or what ?!!

Try a different electrical outlet.

"two weeks ago"

If changing electrical outlets doesn't help, if it's beyond 14 days from the date your received your Mac for an exchange as noted here, your Mac is covered under the one year limited warranty including the USB cable and adapter.

Make an appointment at a Genius Bar

How hot is hot?

Install a temperature/fan speed monitoring application. Open Activity Monitor to ALL PROCESSES and set CPU to display values from high to low.

When the MBP gets 'hot', post images of both for inspection.

Ciao.