The fact that it is an ISA means it's not so easy to pocket the cashback and run.
You can't have contributed to another S&S ISA already this tax year (or that would have to be transferred too as I understand it). Monthly contributions for 3 months would take you into the next tax year, so you couldn't contribute elsewhere without transferring. I suppose if you're not already using ISA allowances it might work.
Ahhh I see not so easy then, I will probably just leave it ....