I think in my mind I have drawn a distinction between safety requirements, that I accept not all GPs or even parents may know about, and general preferences over diet etc.
Non negotiables, such as cutting grapes, car seats, food allergies should 100% be spoken about and reinforced, just in case the carer (whether it be school/nursery, GPs, aunty, friend etc who is providing the care) is ignorant to these issues.
Preferences over diet, activities or clothing for example, which don’t have serious, immediate detrimental effects if deviated from occasionally, should not need to be stipulated and to some extent, I can imagine carers (be it school/ nursery, GPs etc) sometimes disregarding such preferences. The difference is that a professional setting will have the confidence to say, ‘Sorry, we cannot guarantee your child will be put down for a nap at exactly 12 noon, or drink a particular type of drink which is different to all the other children,’ unless there is good cause to.
There is clearly debate over whether a cup of milky tea of harmful and I have no reason to add to that, as it’s not really the issue here. I am just guessing that OPs request was taken as a preference by the GPs and as such, they decided to use their own discretion when making the choice in loco parentis. This is an error on the GPs part (given what they should have already gleaned about the OPs parenting style) and something that OP needs to communicate further with them about, telling them how serious she thinks it is. OP herself states that she is not sure that GPs took her request seriously and questioned whether she should be more forceful.
There is disrespect both ways in the current scenario, GP disregarding OPs request and OP failing to recognise the important contribution that GPs are making to her daughter’s life and maybe even considering further reducing contact over milky tea. Would the OP raise an issue with nursery if she felt the cake portion size was too generous or she if disappointed that her DD came home dirty? It may be telling that the GPs have not pushed back on what they possibly see as an overbearing request - and it’s clear they are not alone in this respect. I wonder if the OPs own parents have a better line of communication with her and more confidence to challenge anything they think is unreasonable. We don’t know the dynamic in their family relationships but from OPs post, it seems that DH didn’t challenge his parents over the tea on collection. If it is an issue they both agree on, surely this should have happened?
Of course OP is not unreasonable to feel slighted that her request has been ignored. What she does with that feeling is a different question.