I suspect that the dress code will relax somewhat, when Charles becomes King, and more when William does. I think that the monarchy has got to change and modernise, if it wants to stay relevant and popular - and I know there are people who don’t think it is at all relevant or popular now - if it doesn’t change, their ranks will swell, and the move to get rid of the monarchy could become overwhelming.
I do think the monarchy has changed and modernised a lot during the Queen’s reign - but I suspect that, at her age, she doesn’t want to change much more, if at all, and feels she has made enough changes during her lifetime, so prefers to maintain the status quo, and leave more change/modernisation to Charles and William.
@Bloodypunkrockers - your question “what do you know that the rest of us don’t?” - about George becoming King - my answer would be that, unless something changes (the monarchy is abolished, he passes away, he decides to abdicate because he doesn’t want to be King) George is third in line to the throne, so after Charles and William die, he becomes King. Given that it would mean a big change in public opinion, and some major legislative change too, I would be surprised if we became a republic any time soon. That is just my opinion, of course, but I am not seeing a major public movement to abolish the monarchy.
I do think he could have worn an England shirt, and I suspect that would have been a very popular move. I wonder if his parents would have done this, but we’re constrained by the Queen’s rules - he was there as a working Royal, so had to follow the dress code.