ITV Question

You can still have a ECP and make the credits interesting to look at though. I.E. Taskmaster on C4, Pooch Perfect on BBC 1.

Yep, being formatted for ECPs doesn't make BBC or Channel 4 credits all be in the same, bland, style.

It's always nice to see some unique takes on the required formatting, I know most channels require their template to be followed so squeezes or towers can be superimposed over credits without obscuring text. Occasionally you see a show which follows the format but at least has something visually interesting like gently blurred footage underneath the credit roll. Makes a nice change. C4 and C5 seem to play with this occasionally.

To OP's question - cue dots were most frequently the black/white rollers, but technically they could be anything. Engineering tests can use very unintrusive subtle indicators, either to check an opt or ​standby broadcast chain, rebroadcast tests...

As others have said, you can flag in the non-picture lines of a broadcast signal, but the entire vision chain must support it and not strip all non-picture lines content. From the studio end, there are scenarios like taking a network feed for a local opt but needing to pass network subtitles while soft opted (in circuit, but still outputting the network feed through the local vision mixer).

In this case, you need to ensure that things like proc amps - or the vision mixer! - don't strip data from non-picture lines. Kit which is capable of SCTI/VANC manipulation can be useful if you need to work around problematic kit in the signal chain.

Nowadays local insertion of cue dots or information in non-picture lines for stuff like opts is becoming less crucial. For example, either in non-picture lines or metadata, one solution for insertion which bypasses 'local' facilities is to remotely trigger kit on the distribution side over a WAN. You can also remotely feed the data you want inserted, post-gallery signal manipulation can simplify system design.

Before the ubiquity of IP networks, triggering could have been done with a point-to-point serial, ASI, E1, even tones over an analogue voice circuit...

An example would be remote control of routers/switchers/DAs at a central distribution facility, to do things like insert regionalised ads/content to produce TX variants for syndication or different platforms, selectively 'blank' programme segments, all sorts.

Regional TV opting infrastructure at ITV is centralised, which enables their HD regions. The BBC will be implementing central opting, albeit differently as they have many more regions and a wide disparity of broadcast equipment. See discussions passim on TVF and DigitalSpy about that.

For those who haven't seen Charlie Brooker's take on credits...

All true, and sadly - nothing has changed.
I missed that programme. Not going to happen since he fell into the Netflix machine, but it would be nice to think he would do another series of Screenwipe.