I suspect that it is hard to integrate after market cruise controls with modern ECUs, unlike the old days. I am happy to stand corrected - its worth googling the question.

I use Cruise Controla fair bit, but usually on quiet motorways north of Manchester. You get a slight economic disadvantage from CC over your right foot, but the increased relaxation makes it worthwhile.

See if you can find the 2003 or 2004 Top Gear programme when JC drove an Audi A8 TDI V8 from London to Edinburgh. The difference in running consumption was about 10% worse using CC.