Back to the original question from DP. The first advice given by FFF is only correct for those airfields in class G which have an Air Traffic Control Service. These are perhaps a minority of airfields. Correct at these airfields you do have to have clearance to taxi, take-off and land.
However many airfields are either non radio or air to ground only. Here you do not need a clearance for anything. The airfield will have standard procedures which are probably published in the various flight guides or you can be told of if you phone up (some are PPR others are not) some will have standard overhead joins others will have diferent arrival procedures. The radio operator will probably only tell you the runway in use, circuit and QNH/QFE and then give you the wind when you are about to take-off/land. Seperation is up to you and consideration for other traffic.
There are often problems at these airfields with pilots who dont know what they are doing and ask for departure clearance or landing clearance which the controller cannot give them, they cannot even say "land at your discretion". You have to make your own decision. Air to ground airfields will use callsign "XXX Radio"
There is also the strange UK phenomenon of AFIS airfields, where the radio operator provides Flight information. This is similar to "air to ground in the air", but on the ground you are supposed to get a clearance to taxi. This type of service seems to provide even more confusion, especially as some of the radio operators tend to try to be controllers sometimes at some places.
They would be callsign "XXX info"
Someone asked what the point was of having an ATZ at somewhere where there was no ATC, just air to ground. This is obviously to "protect" the aircraft in the circuit, so they can go round the circuit without having to worry about tranist traffic. Without an ATZ anyone could fly through the middle of the circuit at 500ft. Also in an ATZ all turns must be in the same direction as the circuit which makes things a lot safer.