Quick update: I used the Wayback Machine (archive.org) to find the webpage for my bike from 2015. It is listed as Di2 ready!!

https://web.archive.org/web/20150311...rbon-3-ultegra

And there you have it.

Not the best angle ...

When was the last time you rode a bike with eTap?

Looks like it'll be most cost effective to purchase all the required components separately. Only thing I'll need to figure out is how long I need the electrical wires to be - I'd be going for the D-Fly as well which I believe adds an extra wire.

Since my LBS would be doing the labour, I'll obviously take them the list of everything and see if they'd be willing to compete (or at least come close) to the online UK shops.

Unlikely they can or even be willing to compete price wise. Also I suggest doing it yourself, you'll learn more about Di2 installing it yourself. And there is plethora of resources to help you. Should be much easier since your frame was built for it. Have fun. Hopefully you do it yourself and then you can be of help to others. Already in this thread, a number of members have said they have built multiple bikes with Di2, if you have any problems, no doubt they can help with any troubleshooting.

My LBS is definitely "willing" to come close if they can. I have a good relationship with them and have referred lots of business. If they can come close, I know they will. If they can't, they'll flat out tell me to buy the stuff online and just charge me for their time (if I use them for labour).

As for getting to know the Di2, I will definitely learn the "technical" aspect once its installed, but not sure I want to install the whole system myself.

The intended assembly is a Junction Box B around the crank area and a Junction Box A around the cockpit area. With that, it should be straightforward to measure on the outside of the frame and add a bit for safety.

As far as I can tell, as long as the components are connected in some way or another, you're good.

NBs:
- the shifters on 6870 have three ports each (it's possible one is reserved for satellite shifters, though)
- you probably want the internal battery in your seatpost, in which case you probably want to have enough wire to pull the seatpost out and disconnect the Di2 outside the frame

Personally, I run a 1200mm from the B directly to the A in the handlebar end (2-port), then a 350mm to the nearest shifter, and probably a 650mm from there across to the other shifter. If I remember correctly, I use an 850mm wire for the battery. The FD is on a 350mm wire and I can't remember the RD specifics. I do not use D-Fly.

EDIT: 58cm frame; 12cm stem; 44cm handlebars.

If I'm correct, with the D-Fly, does the wire that normally connects to the RD connect to the D-Fly instead, and the D-Fly connects to the RD with another wire?

You can do it how you want, I think. In the official Shimano material, the new D-Fly sits at the cockpit. I think that's partly because the new D-Fly is much smaller than the old one and partly because it allows for just one description of how to do the connections, independently of the type of Jct Box A, etc. I would imagine it's also to "encourage" people to buy the unit.