Ametyst1,
I totally see your point about doing LPL - DUB - LHR and on for a host of reasons. If you like flying this makes sense, you get avios on all legs with AerLingus and BA..
Moreover, BA has pricing strategies by country. Dublin has had some very preferential deals of late as reported online, offering cheaper upgrades and bonus miles. So it may on occasion be much cheaper to start your BA trip in Dublin over Manchester!
A pal of mine who lives in London went to Delhi. He did club world but the deal was cheaper from Brussels, believe it or not than London. The offer was cheaper with reduced level of avois required. So he flew LHR BRU to meet his flight... Which took him BRU-LHR then on to India. I don't think this is uncommon. My time is not so previous thst I can't spent an extra few hours doing something like this especially if I am said a few hundred quid.
There many bizarre pricing polices in some airlines like more expensive for 1 way than return eg the very successful AirFrance!! And some airlines have pricing policies in different countries that don't at the face of it make sense compared to direct flights, but if you compare and be a bit forensic about it you can save money and probably bag lower prices or bonus air miles or indeed discounted upgrades.