I noticed today's DTL with Nigel Farage appeared to have absolutely no audio delay, allowing the debate to flow much better. Is this just because Nigel was in a Westminster facility I wonder.
Almost certainly. If a studio is sent via a fibre or similar resilient, IP (ideally not internet) route, particularly if very lightly compressed, or uncompressed, the delay can be just a few frames (a frame is 40ms).
If you need to use DVB or VSAT satellite (which has speed of light delay), low bitrate codecs (which introduce codec delay) or mitigate packet loss (by buffering which introduces delay) then you get much more delay...
The ITV internal video distribution and data network called XNET is very low latency, a frame at the most. I didn't see it but the DTL was probably from the ITV/ITN/Channel 4 shared facility in 4 Millbank which is connected to XNET. Reverse comms would have been done over POTS or ISDN which (depending on ISDN codec used) improves overall latency further.
I work in telly, I sometimes get time to watch telly. More of a technology geek than a presentation geek!