There's a legend that a man called Simon and his wife Nell argued over whether the cake for Mothering Sunday should be baked or boiled. My first thought was, "Ew, how could you boil a cake?"; then I remembered that bain-Marie thing, so I guess that's prolly how. I don't think I'd consider it really a cake, though.

The name Simnel probably comes from the Latin word "simila" which means a fine wheat flour usually used for baking a cake. See, from Gurunet:
sem·o·li·na (sĕm'ə-lç'nə)
n.
The gritty coarse particles of wheat left after the finer flour has passed through a bolting machine, used for pasta.

[Alteration of Italian semolino, diminutive of semola, bran, from Latin simila, fine flour, ultimately of Semitic origin.]

The name of the cake makes me think of the word seminal, which I think fits, in one sense at least.