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Say I want to engage in boxing lessons for 3 days a week. Will this be an expensive Endeavour?

No idea. You'll have to ask around at gyms in your area. Prices for martial arts schools and boxing gyms vary wildly.

You generally get what you pay for - the more the instruction costs, the better its overall quality is likely to be. I don't know what the situation is in the UK, but here in the US, boxing lessons that involve actual ring work have become popular with yuppie corporate types

The serious amateur and professional boxers still tend to come from poor backgrounds, but those with talent are identified and then managed. They don't pay for their lessons.