Merriam Webster-
"learnt Chiefly Brit past and past part of learn."
OED-
Learn v pa t and pple learned, learnt.
In other words both are acceptable to the OED if not to Nigel and his teachers (I think I was taught the same as you, Nigel, but we end up absorbing the teacher's preferences).

Learnéd in the sense of erudite.
Some Americans speak of a knowledgeable man as learnéd, using the two-syllable pronunciation used in the OED, but Webster's accepts both, reflecting the American tendency to abandon the accented -ed ending.

Yep, still biting. John H, I know that in talk I probably mess up light-lit- lighted. Webster's accepts lit or lighted as past for the verb. Lighted is often seen in 19th C. writing. In the US Navy they have the phrase "the smoking lamp is lit."
Lit often used with up- her face lit up. Lit (up) = drunk. Lit as short for literature.
Webster's also accepts both "the bird lit on the branch or lighted on the branch. I know some birders who prefer lighted but I think most of us say lit.
The OED also accepts both. In the US I think most of us turn on or turn off a light bulb or lamp and avoid the l-word. Or we could outen the light as our Pennsy. Dutch friends are supposed to say.
Hmmm, bite, bit, bitten (and bit)- why not light, lit, litten?

Lighten up?