Hi Jon,
Personally, I don't think there's anything you need to apologise for.
There's no problem I can see here, especially not one that requires any apologies or formal policies or long drawn out email threads to debate it.
You donated your time to provide constructive, helpful input in response to Rosie's question. Personally, I think that's something positive, not something anybody ever needs to make apologies for.
Mentioning a Freetronics product as part of your response is no different to mentioning any other product available from Jaycar or Sparkfun or Adafruit or Little Bird or whoever it is, in my opinion. Obviously you would expect people to name-drop several vendors and companies in the usual course of writing a constructive on-topic reply to a thread like this, and that's sensible, productive, and I can't see how it's unusual or harmful at all. It's difficult to write a constructive helpful response to a question like this without name-dropping at least some shop or vendor for hardware. In fact, I would consider Freetronics to be "better" than anyone else such as Little Bird or Jaycar or whatever, because all Freetronics products are of course always open source - although they're usually not open-sourced during preliminary development, only after the product is designed and finalised and released onto the market. There are other advantages too, such as supporting local Australian business.
It doesn't make sense to say, well, this post should have been moved over to a separate mailing list or a separate thread or something, because it's an on-topic productive response to Rosie's question, so it makes sense to post it in this thread, on topic, where Rosie and anyone else can find it, and not to post it anywhere else.
If it was something explicitly commercial such as a commercial advertisement for a certain company's products or a recruitment post looking to hire somebody, and it had no relevance to answering anybody's questions or replying to any existing discussion in an existing mailing list thread, then sure, it would probably make sense, as Andy suggested, to have a separate thread or a separate mailing list dedicated to that purpose so people can easily filter out that content from their email as they deem appropriate.
Basically, I think you've got nothing to apologise for and there's absolutely no problem here that's worth worrying about, but that's just what I personally think, and of course this is a democratic community and what I personally think has no special weight any more than what anybody else thinks.
Regards,
Luke