Be aware that the action of getting clients to buy materials to keep your turnover below the threshold could be seen by HMRC as deliberate and they may attempt to use the anti-avoidance legislation against you. They'd have to prove their view on this in court.
I'm not saying it is wrong, just don't mention your clients buying materials thing as it might ignite HMRC's flame a little. With regard having 3 months off, this is quite common practice in the trade (they go work for someone else for three months and earn a wage) and is an acceptable reason to give HMRC.
Despite all that, you may have to show HMRC that the last 'job' involved unusually high amount of materials hence pushing you over the limit but either way, it may attract HMRC's attention to the business.