1. Oxygen. In the US it's mandatory above 12500ft, whereas in Europe it seems to be 10.000ft. Is this European one a recommended altitude for oxygen, or a mandatory one?
European regs for what one might call "equipment carriage" are listed in each country's AIP, and the full matrix is going to be a dogs dinner.
In the UK, the ANO now has some stuff on oxygen. Look
here and search for "oxygen". It's there. I think this reg is current.
2. Transponder. What exactly is the difference between a Mode C and a Mode S transponder?
Gosh, a potentially long explanation. A mode S one transmits a 24-bit number which links (via a database) to the aircraft reg. In a high end (non GA generally) installation it can also transmit stuff like autopilot settings.
3. Transponders abroad. Was it in the Netherlands that one could not fly without a Mode S transponder? Any other countries that have similar rules?
VFR or IFR? For IFR, a mode S is a must, everywhere, more or less. VFR has various examptions but they are so complicated that unless you just bimble you may as well get one.
4. Weather. From when I read TAF's and METAR's, I can't find any references to the tops of the cloud bases, only the ceilings. Where do people get this info?
Oh dear, another one
I will sedn you a PM.
5. NOTAMs. Where do I find them for, let's say, international flight planning? And is there a way of plotting a route visually and then have all the relevant Notams that could affect pop up? If not, how do you sift through all of that to find relevant stuff pertaining to your track/route?
Here. To cut out the crap, you use your head. It is pretty easy. Nearly all notams are irrelevant (like, telling you that 3 RAF planes will be flying awfully fast somewhere...)
6a. Min altitude. Back in the days when I did it in Sweden it used to be 500ft AGL was the lowest altitude one could fly at except for approaches and takeoffs. I couldn't see any reference to that in my new PPL theory book. What's the minimum alt?
Same here, 500ft spacing to people, houses, etc. 1000ft above towns, plus you (if SE) must be able to glide clear.
6b. Min altitude at airport. This question only relevant if min altitude is 500ft AGL (see above). At Lydd they won't let anyone take-off VFR when the cloud base is below 1000ft. Now, if the min altitude is 500ft AGL allowable, then why do they impose this? And is it up to each airport to set their own minima (not saying it would be prudent to takeoff in 500ft of cloud base, just wondering about the legalities)?
ATC control things and they can do what they like. If you think about it, a departure in OVC006 could be legal so how will they stop you? At a NON ATC airport they can't stop the movement anyway.
Airports in CAS (Class D in the UK, basically) have a min cloudbase for VFR, generally 1200-1500ft but it varies. This is ICAO (1500ft I think).
7. What to call them? When you approach, let's say, London TMA and call them up to get a clearance - how do you know what to call them? How do you know it's Control and not Radar, or Approach? All it says is London TMA or CTA or something on the map.
The LTMA is Class A and is therefore no go for VFR. You can't go there IFR either except on a Eurocontrol route, which is altogether different (FL100 min, generally). Big subject...
8. Permit To Fly. What's the difference between a permit to fly and C of A in practical terms? Can you fly abroad with a permit to fly aircraft (seem to remember that Jersey wouldn't allow PtF aircraft)?
PTF is national and has no default international flight privileges. Also no IFR.
9. Transponder settings. It seems like most instructors want you to put the transponder to STBY after checks, and then switch it to ALT before you leave earth. Why is that? Why can't one just leave it in ALT all the time?
You can leave it on ALT, but if you do it at a radar airport they might not want so many returns. OTOH at some (Prague is one I think) they actually want you to leave it on.... A good point.