I think it's more of a cost issue. Continuously operating and maintaining 2 runways is more expensive than one. In the winter time, 12/30 isn't used at all, which saves a fair bit of money I presume.
What normally happens during busy times in the summer (mornings between 0800 and 1200) is in a northerly configuration, landing will happen on 31 and take-off on 30. There is no instrument approach to rwy 30. Coming in from the North, so in the southerly config, landing will take place on 12 and take-off from 13.
When you talk about adjusting 13/31 to a new 12L/30R, you have to think about the surrounding terrain as well. Quite a lot of high terrain to the north, with not that much room to manoeuvre. I think the way they handle it now is quite okay, at busy times 1 runway dedicated to landings and 1 for departures, works out quite nicely. LGW would be jealous of such an arrangement
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