1. Nothing to do with level of activity - it's to do with whether it's licensed or not. Licensed airfields have an ATZ, unlicensed ones don't. Some unlicensed fields, such as Popham, can become much busier than certain licensed fields, even though they don't have an ATZ. A bit of local knowledge is required to learn which un-licensed airfields are busy enough to stay away from...
2. Kind of, yes. You're not allowed to enter an ATZ unless you have enough information to be able to do so safely. This is usually interpreted to mean that you inform that A/G service that you intend to overfly the field, and they give you all the information you need, which would usually be, at the very least, which runway is in use, which direction the circuit is, and what the local QNH (and possibly QFE) is. If you don't get this information, you're not allowed in the ATZ. However, since they are not ATC, they're not allowed to not give you the information if you ask for it. So, in effect, you're right. Many airfields will also give you traffic information, but bear in mind that the radio operators are not qualified in any way, so their assessment of the traffic may not be the same as that of an ATC station. (On the other hand, the person working A/G may be an ATCer on his day off, who's able to give you quality information - you just don't know.)
Hope that helps,
FFF
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