Consider that London has a population alone in the region of 12 million, or so. An incredibly wealthy city, with huge premium demand. Plus the rest of the UK (via air, or rail, or motorway) plus BA's European, Middle East and (Indian)Asian network to allow connections to/from. Only 4 flights a day seems not very many!
DUB is a city of 1m + 3 million more (in good times) for the rest of the country. It is a very low-yield, price sensitive market with poor Business Class demand. There are very few connection opportunities ex DUB. Although cargo demand ex Ireland is probably increasing and USCBP are factors in favor of the service, it is still against a backdrop of very weak travel demand.