I believe that it is an upgrade to the access equipment and software, whilst the full CLI is already present you only get sent the first (15?) characters including spaces, a full international CLI could be up to 22 digits. This will span 2 lines of the display unit, at present we only see one line.
As this is a software upgrade it is likely to cost tems of millions of pounds for each type of switch, I wonder how many types BT has? I suspect that the switches haven't had updated software for years as BT were going to replace them with 21C at some point and updating the software on old code ( circa 1990 ) could cost a lot if only to train up people to do it.
You also have to have the equipment to send the signals to the line on each call, the more people that have the facility the more this will cost. (On a mobile you have different signalling to the phone that can carry the CLI within it.) The info comes down as a hidden frequency before the first ring ( on my wired phone you can hear it 'click' before it rings) so that it doesn't interfere with the ringing.
A google search give SIN 227
which is far too technical for me to understand it all
Thus this is not free
Personally I prefer the "International" we get today, it enables me to list and replace the receiver without much thought.