When do I start preparing for bmat and ukcat?

People prepare in different ways because they have different ways to learn. But generally I would say:

1 - It will take 3 weeks or so to get yourself to the right level where you can confidently do the exercises, understanding what is required and finding the techniques that suit you.

2 - It is best to do a little bit every day rather than hours every day because it is quite boring in the long run and it will seriously do your head in.

3 - You should start with untimed practice and then move to timed when you get to about 10 days before the exam.

4 - Don't waste your time with Decision Making as that section is not marked this year.

5 - Expect your first few mock exams to reveal low scores. It happens to all of us.

There are several resources available. The official UKCAT questions on ukcat.ac.uk are obviously very valuable and I would suggest you do one test at the beginning to get an idea and keep the rest for the end of your prep.

TheMouseS is right to say that the ISC 600Q book is out of date. A new version with 1000Q was published this year which is fully up to date and many people use it to acquire the techniques. But it is harder than the real exam and so you should use it for your untimed practice.

For your timed practice, most people use Medify. Just get a 2-week subscription. That should be sufficient.