I don't think you'd go wrong with any of those four malts.
However, if you're trying to closely adhere to the style guidelines, Golden Promise is probably the best malt, with FMMO next, regular Maris Otter, and lastly the Mild Malt. Because a Southern English Brown highlights the sweetness of the malt, with more subtle accents of side flavors, like caramel, toffee, and nuts, you want the sweetest, cleanest malt for this style. The difference between Maris Otter and Golden Promise, the Scottish version of Maris Otter, is pretty minimal, but the Golden Promise has the edge on getting sweetness into the flavor and finish.
Maris Otter will produce a nice toasty nuttiness, and the floor-malted version will be slightly more complex, but both MO malts will put too much focus on the nuttiness and pull attention from the sweetness. I believe Maris Otter would actually be the traditional malt for a SEB, but Golden Promise is generally a good substitute for Maris Otter.
Lastly, Mild Malt will give you more dextrins in your malt, which will lead to good body and mouthfeel, but again, the sweetness will be lacking.
Again, any of the malts will make a fine SEB, but Golden Promise is the best malt for the style.