Welcome to Flyertalk and greeting to the BA forum lightlamp4, you are very much welcome here and I hope you will continue to participate here.

The short reason why there is such a thing as a Saturday Night Rule is to distinguish between business travellers - paid by their companies - and leisure travellers. Now many corporates are very much aware of this and therefore don't mind if their staff travel a day early or late to get the Saturday Night in their travels, but that's the essential reason. It doesn't apply like that to all sectors, but ICN would be very much one where this would apply, there isn't much of a leisure market for UK to South Korea, but the business market is very much there.

Work arounds include starting from other places apart from the UK and also booking an inclusive hotel or car hire package via BA Holidays.