An Unmanned Aerial Vehicle (UAV) is an aircraft that has the capability of autonomous flight, without a pilot in control. Amateur UAVs are non-military and non-commercial.  There are also more simple semi autonomous UAVs that will automatically keep the aircraft stable, requiring minimal guidance from the pilot

To build a drone, you dont need to have a fancy degree.  You only need to know how to read,  but since you found this website it is safe to assume you can.  There are many tools available today that will enable you to build your own drone with no programming or electronics knowledge required!

The simple answer is yes and no.  If you are prepared to build most of the stuff yourself, you can build a very simple drone/UAV for a under £200.  However if you want to have extra fancy bells and whistles like live telemetry or fancy sensors things can get more expensive.

There are many possible reasons for having your own UAV.  Maybe you just think its cool to have an aircraft that can fly itself.  Perhaps you want use it for a more commercial application like aerial mapping or photography.  Others may want to put it into their expensive RC model to reduce the chances of a crash, or help them learn to fly.  

It is generally acceptable to own your own drone.  However there are some guidelines that you must follow.  It is also important to check with your local aviation authority as it may not be legal in your country.   Here are some UK specific UAV regulations