It's not grey at all, it's black and white.
Who exactly would go to the expense of travelling, getting the visa and returning for the sake of a few hours training? It would be cheaper just to finish in the UK, if that were the case. Your story makes no sense.
If it's a full time course of study (more than 18 hours per week) and the primary purpose of your visit, you do need a visa. Otherwise, you can use the visa waiver program.
This stuff isn't complicated. The rules are simple and well spelt out. If you don't understand, you shouldn't be giving people advice or criticising them for doing something entirely legal.