By law we must fly within Visual Line Of Sight (VLOS) which means that the aircraft must be able to be clearly seen by the person flying it at all times when it is airborne. The maximum distance from the pilot is 500m - although a vehicle can be used to move the pilot with the aircraft.
In simple terms, the aircraft must not be flown out of sight of a human eye.
We are also limited to 400ft above the ground, however, it is sometimes possible to get permission to fly higher. Let me know what you are planning and I can have a chat with the CAA to see if it's possible.