Hold off for now, you may not need it. I trialled it over Easter Y12 for maths as I was struggling a bit with mechanics. It helped a lot and I thought I might pay for membership from the start of Y13 or possibly Christmas (the price goes down in intervals as the year progresses). My mum talked to them and they gave us a £50 discount so we paid the Y13 price but got the rest of Y12 for free. I don’t know if they always give discounts if you book a call, but it is worth a try if you are willing to walk away as we were.
I doubt I will complete enough of the course to trigger the guarantee, for me it is more something to dip in and out when I have problems plus exam practice questions and papers. Like a very expensive text book - but it works for me. We had hegarty maths for GCSE, but I think Uplearn is better.
If you want to do something the summer before you start, just make sure your algebra is as good as possible.