Yes, you are quite correct, all (or perhaps almost all) EI services can be purchased via BA.com, just put in the route into BA.com and up it will pop. I have made a number of these services now. There are a number of threads on this, here are a few for you to peruse:

http://www.flyertalk.com/forum/briti...ou-future.html

http://www.flyertalk.com/forum/briti...er-points.html

http://www.flyertalk.com/forum/briti...s-dsa-ema.html

This comes essentially in two forms: If you buy a simple flight like EDI-DUB then it's unlikely to be cheaper as a BA codeshare. If you buy it in collaboration with other services it may be cost effective. And there are some wierd quirks too e.g. LPL-DUB is relatively cheap on BA as a one-way, DUB-LPL relatively expensive, LPL-DUB-LPL is middly to expensive, MAN-LHR-DUB-LPL can be quite cost effective.

You will find the TPs in BA.com / collecting Avios. Put BA in if codesharing, not Aer Lingus, and you get between 5 and 20 TPs depending on fare bucket. In theory 40 TPs for business class but that's not offered on that route.