You can disagree if you like
Well that's jolly nice of you! Thank you.
but it doesn't mean you are correct in what you are saying
Absolutely. It is an opinion and I haven't phrased it in such a way to say that you are wrong and I am right. I have always been happy to change if someone can show me the error of my ways. That's how my driving & coaching skills have developed over the years.
and you should look at what you're doing carefully. If it is still suggested on training runs then I would suggest said tutors/instructors look at what they are doing. If it is still looked for on tests by some IAM/RoSPA Examiners, then I suggest that said examiners get themselves up to date.
Oh! Is there something out there that says examiners mustn't allow offsiding? I'm not aware of anything. As I mentioned earlier, offsiding isn't recommended in Roadcraft, but neither is it forbidden. It is something I rarely do but that isn't to say I would never do it.
Went for a run recently with some Pistonhead forum members and some were happy to drive using whole road to make better progress - not correct to do it but if they wanted to do it to push their cars on, and at times, above the limits then that was up to them and they were aware of the consequences of any additional risks they were bringing into their drive. But .... and a big but, I wouldn't expect them to do it as a driving instructor and say it was ok to do it, never mind coach instruct other drivers to do it either through an advanced driving group or any other means!
Crossing the centre line when there isn't a solid white line is not wrong, breaking the speed limit is, however and I wouldn't condone it except under exceptional circumstances.
Now, if the road was not marked with centre lines, thats a different thing and it is ok to use the full width of the road but again you need to be aware that you don't confuse following or preceeding traffic and don't put yourself at more risk at areas of restricted visibility. On narrower roads there are clear and distinct advantages of doing so when there is no room for two cars to pass.
Do you really feel that you need to spell this out on this sort of forum?

Some are tip-toeing round about this but here goes without any malice intended:- While I appreciate the qualifications you have listed Nigel, I do think the hallmark of a better instructor is sometimes to take it on the chin and admit that heh . . maybe sometimes I have been misguided and misjudged or even got it wrong or heh .. . things have changed a little and this bit is important so I will now change what I am doing or teaching!
Oh right! So I'm wrong and you're right? So that must mean that my RoSPA Diploma trainer must have been wrong, the chief examiner must be wrong, the local RoSPA & IAM examiners must also be wrong and all the local observers too.
Driving is such a large grey area that there are many ways of doing things, none of which are necessarily wrong. I think you need to add some flexibility & adaptability to your approach.
Why would a professional instructor dig his or her heals in to drive (and maybe sometimes teaching) in contravention of a 'rule' in highway code (crossing centre lines when not overtaking or turning right) and also in contravention of 'guidelines' set out by IAM, RoSPA and other Fleet providers?
Dunno! Why are you digging your heals in?
I am happy to leave others to develop their own style of driving. I don't try telling people what they MUST or MUST not do. I may suggest reasons why I would or wouldn't do something. Flexibility & adaptability are key!
...A little confused by this

as broken white paint is not a solid obstacles, however I agree that IAM and RoSPA trainers should have firm guidelines on what is and is not acceptable when teaching a member of the public. They are essentially using tools in the tool box that are not designed or necessary for their use.
Are we?

What an interesting accusation.
Leave it to the emergency services who receive specialist training on such driving techniques with the purpose of enabling them to attend incidents as quickly and safely as possible.
and a condescending comment to finish off with!

Nigel