I am a UK citizen and I have a training company here in the UK. A company in Canada wish me to visit them and offer a one day training course. What work visa would I need for this? This company are unable to find this training within their own country.

Also I would like to be able to offer the training in Canada without being first hired by a company in Canada. For example there are many freelance persons who wish to complete my training course. Would I be able to hire a venue and gather several freelance people together and offer courses? What kind of work permit is required for this?

I would only be working for a few days at a time - maybe 4 or 5 times a year.

A Labour Market Opinion and work permit would be needed, even if the work is only for a day. You could not pursue a work permit as self-employed in order to pursue contracts; a work permit could be sought only once work is arranged.