True, but the ANO is not applicable when flying an F-reg. When you fly an F-reg it's the French equivalent of the ANO that is applicable. Under JAR-FCL, there will be something in there similar to the following in the ANO:
Subject to the exceptions set out in articles 51 to 60, a person must not act as a member of the flight crew of an aircraft registered in the United Kingdom without holding an appropriate licence granted or rendered valid under this Order.
- and -
A JAA licence is, unless the CAA gives a direction to the contrary, a licence rendered valid under this Order.
(ANO part 6, para 50.1 and 62.5 respectively - I'm sorry but my French is not good enough to search for the actual French article.)
This means that your UK licence is now considered the equivalent of a French licence, and is thus valid for flight on an F-reg. But this also means that the DGAC rules apply, and that the DGAC can give you additional privileges and set additional restrictions if they want to. After all, you are now flying under French law, not under UK law. Normally they will simply state that all restrictions on the original licence apply, and will not confer any additional privileges, but my point is: They don't have to do that. It would be perfectly legal for them to say "Hey, you've got a UK IMC rating. Congratulations. Based on that, we'll give you a French equivalent of that, valid in France only, even though your UK IMC rating is only valid in the UK." (Well, they would be saying that in French of course.)
Now I've never heard of the situation where the French authorities actually did this. So I'm very interested in hearing about these. (The reason I find this very unlikely is that there is no French equivalent to the IMC. So embedding such a validation in the rest of the legal framework, and dealing with such a validation on a practical, day-to-day basis, would be too complex, IMHO.)
The exact same thing, by the way, happens to the UK NPPL, Dutch RPL and various other sub-ICAO sub-PPL licences. Yes, these licences are only valid in national airspace and in an aircraft registered with the national authority. But a foreign country can issue a generic or specific law or exception, to accept such a (foreign) licence as the equivalent of their local licence.