A website called How2Become has lots of practice tests, you will have to pay for it but I guarantee you it will really help.
I bought the Armed Forces Recruitment Tests book from How2Become and I very glad I did. I don't think there are any questions you will come across in the actual test that aren't covered at least in similarity and difficulty.
I would recommend buying GSCE revision books on anything you're a little worried or rusty on out of the English, Maths or Mechanical (I guess, physics?) and going through it once a night until the test. I also highly recommend doing some logic puzzles too if you're prone to overthinking.
The actual test is very straight forward and rather basic. Thinking too hard will lose you time, so relax. As said before, if you don't know the answer to something move to the next question and come back to it once you've completed the others.
If you do skip a question make sure to move onto the next answer box as well! It sounds obvious but I spent a few hours in absolute horror after the test as I skipped some of the questions on the maths section only to find at the end I had, in a panic, not moved to the next answer box. After that shock I repeated it on the Mechanical knowledge section. I fixed them in the last minute and I passed but probably by divine intervention.
You won't be told how well you did (or at least we weren't). It'll be pass or fail.
Don't hope, or wish for luck. You only need your knowledge to pass. Relax though, it's not as hard as you think it will be.