Are you in the Colchester grammar schools/Southend area or Redbridge? (My geography isn't too good!)
I think it's fairly competitive around there.
Yr 5 is the usual time people start familiarisation.
What you will do depends on the exam (VR? Non-VR? Is there a Maths or English paper?).
It's true that you can tutor yourself. As has been said on many other threads, there are a few techniques for speeding up VR and NVR, which you can teach yourself. After that, it's regular familiarisation to build up speed.
You may be suited to teaching that yourself, or you may want to get a tutor. Likewise, if you want to/need to do anything about maths/English it's possible to do it yourself but you may prefer a tutor.
Ime - a huge number of the dc taking the VR and NVR exams cluster around a quite narrow span of marks, so a lot of it does come down to speed and accuracy. Being very quick at times tables is a very helpful skill.
Have a look at the elevenplusexams website, which will give a good deal of information about the types of exams in your area (if you don't already know) and also those "techniques".