Ah right. I have always had broadband so most orders would be migrations. I don't remember my first install: possibly Virgin Media or it's predecessor; so not actually ADSL.

I moved from Uno to BT during lockdown. It went very smoothly.
Whilst I could not choose a date when ordering it was very easy to change the date online after the initial date was advised to me to almost any date of choice.

interesting.

Looking at my current order all I see is :-

"Current step.
ACTIVATING YOUR LINE
Broadband orders typically take a week to complete. Your broadband order should complete on {strCustActivationDate}. Please note that this date can sometimes change. We'll let you know if this happens."

Nothing about changing the date, which isn't even being picked up by the ISP's system, so something is broken before we even start!

https://forums.thinkbroadband.com/plusnet/t/4656514-...

As the field is ISP specific directly people to same issue, just worded differently.