Hey there

I'd recommend starting with a mock just to gauge how far along the road you are in regards to getting your desired score, regardless of your current confidence levels. You can then map out your ideal study plan (which realistically won't be followed) and adjust as things (foreseen and unforeseen!) change.
Not willing to pay for a UCAT program - there are plenty of resources out there. However, your current proactiveness (which is absolutely brilliant by the way) might stretch your resources a bit thin, so look to target areas of weakness right now before really digging into those resources. I've posted
this link elsewhere a few times, but here it is again, for convenience

. Just a site I used to pop on just for fun, it's straight out mental arithmetic (since you pinpointed that as an area of weakness). Track your scores as well, both to detect discrepancies and to map out improvement.
Again, with the proactive attitude, it's brilliant! It's not completely necessary to get completely stuck in, but I suggest you should (as you've already pointed out) 'try to find a method of doing the questions' that works for you. Free resources tend to provide answers as a bare minimum. I think you should only use free resources that have methods of working out displayed if you're seriously struggling because each approach is unique and if it does end up working and conflicting with suggested methods, it could end up being pretty bad - but that's just my take, so take it with a grain of salt. Do do questions regularly enough because as with anything, you've got to stay 'fit'. Understand also that lectures and seminars have diminishing returns and tend to waste your time after you've gone to a few (but they are free, I guess) - you're much better off devoting that time instead to practicing or your other school stuff which is going to help your ATAR immensely if you're planning on med entry.
Hope this helps
