The cheapest way to get a full PPL might be to train for the NPPL on a SSEP and then upgrade it when you meet the additional requirements. I think start to finish this can be cheaper than going straight for the PPL.
You would need to make sure the flying you do on the NPPL is on a SSEP and not a microlight so that it counts as 'good' time for the purposes of hour building towards the CPL. As Dan Vector says the advantage of this approach will probably dwindle if you overrun on the hours needed for the NPPL.
Every hour you fly in a microlight will need to be repeated in a SEP sadly, so learning to fly a flexwing first will be money down the drain, if you are going to go commercial.