If I understand you correctly then you have been wronged in two ways by your employer in the course of your apprenticeship; you did not get the promised training, and you did not get the minimum apprenticeship wage.
I think you need to decide which way you are looking at this. You can go one of two ways:
You were an apprentice who should have been treated as one, so you want your wages bought up to the apprenticeship minimum and damages for the training you missed.
You were a regular worker who was not paid minimum wage, so you want your wages bought up to the statutory minimum.
The apprenticeship way has the advantage that it was the clear intent of the parties, evidenced by the contract you signed. Its not clear from your post whether the employer was supposed to provide you with training or if you were getting that from somewhere else; this is a key point in the question of how much your employer owes.
You seem to be planning to claim damages for the lack of training by comparing your pay as a pseudo-apprentice with the statutory minimum wage for regular workers. However you should think beyond that; how has the lack of training damaged your career prospects? Can you start again in a new apprenticeship with someone else? If so then this has just delayed things a year and your approach seems reasonable. On the other hand if this is going to cause longer term problems because you have missed a window or used up one year out of a limited allocation (I don't know how apprenticeships work) then this is a much bigger issue.
To put it another way, how much money will it cost to put you back to where you should have been if your employer had kept their side of the bargain? That is how much you should be claiming. For instance, it might be that you could get the training by doing the same number of days study in a block now; that would be around 2.5 months for each year of your apprenticeship. If that can be arranged then your employer should pay for it. There may be other ways forwards depending on your situation; you should find the one that best repairs the damage done to you and then claim for the costs and time (at apprenticeship rates) required to do that.