Summary
Wow - this is great. How many un-related questions can we ask in one thread? Ok, let me try to summarise - based on a combination of my prior understanding, and the previous posts on this thread:
To fly IFR in most of the world requires an IR. In the UK it doesn't, but you do need an IR or an IMC rating to fly in IMC.
To fly an ILS doesn't require anything special - it's just an approach. You can fly it IFR as long as someone on board has an IR (or an IMC rating if in the UK), or you can fly it VFR. ATC may treat you differently depending on your choice of rules.
A safety pilot is required when under the hood in VMC, whether VFR or IFR. (Except in Portugal, apparently, as discussed on another thread.)
Flying VFR on top is allowed anywhere in the world. However, a UK license without an IR or IMC rating prevents it. (Haven't seen anyone say this on this thread, but it is my understanding.)
Logging P1 with an instructor on board is entirely legal if you are legal to fly the aircraft yourself. But only if the instructor doesn't log the time at all. This needs to be agreed in advance. It would have implications on, for example, whether you choose to ignore your instructor's advice on how to manage the flight.
Instructing in France on a UK license in a G-registered aircraft. The suggestion has been made that this is illegal, but no one has been able to back this up with reference to legislation. 2Donks has found legislation which "appears" (his word) to suggest that it is legal.
Have I missed anything?
FFF
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