Adding/moving a light is not notifiable so there is no reason for you not to do the work yourself as long as you deem yourself competent.
I'm assuming the bedroom is on the first floor and you have access to the loft? Pop up the loft and have a look around (don't touch yet) and see if you can get a grasp of the way it has been wired. Find the cable/s going to the light fitting.
If there is just one then you need to trace that back to find the associated junction box (which should have the feed from another junction box/consumer unit, a switch wire, a cable going to the light fitting and possibly another cable going to the next junction box).
If there are 2 or more cables at your light fitting then you will not have junction boxes involved and instead there will be a connector strip in the light fitting making all the connections.
Just follow the diagram that applies and replace the wire going to the light, taking great care to label and make a note of any cable you detach.
Regarding installing a new light: just take a feed from a junction box/connector strip as shown in the above diagram (supply to next...) and then depending on the light and switching arrangement take the to a junction box/connector strip and wire up the light and switch according to the diagrams.
P.S Don't forget to turn off the power and ensure the circuit it dead before touching the wiring. Also make sure you mark any switched live cables with brown tape or sleeving and any bare earth cables should have green and yellow sleeving.
Davy