Were there Peeresses with diadems too? I have not seen television yet. Judging from pictures there were hardly any?
The attentands in red cloaks are members of the House of Lords, then there are ladies and gentlemen seated on the benches, by far most of then in regular daily dress. The members of the House of Commons are standing way behind, just in regular daily dress too.
I think somewhere with the reforms in the 1990's the logica has gone. Back in old days the Queen, the ladies of her House, the Peeresses would all wear long and diadem. That 'uniformity' has gone a bit. Thinking back to last week (Dutch Investiture) the dresscode was white tie (royal Household, royal guests, corps diplomatique) and jacquet (members of parliament, state). However the dresscode was not enforced, it was a pleasure to see the majority following one of the two dresscodes.
Now the weird situation has arisen that there is more pomp and circumstance in the UK while there is a lower dresscode. It are the many red cloaks of the Lords which 'hide' the reality a bit.
Good to see the Prince of Wales and the Duchess of Cornwall attending!