I went on the skills assessment in February and passed. Once you have booked a place you should be sent an e-mail with a breakdown of how the day will go. This pretty much covers everything and there aren't any curveballs during the day.
I used PilotAptitudeTest and whilst it wasn't that similar I do think it helped my to practise the same skills that they were testing. I have heard more Ppruners recommend SkyTest since passing though, so perhaps look there. I also used BBC GCSE Bitesize, and whilst it prepared me well for the maths test and mostly for the physics, it did miss a topic that was asked in the physics test. It was one of the harder questions, which are worth the most points. Unfortunately I can't remember what the question was so can't help you there.
I hope that helps but if you have any questions feel free to ask and I'll try and answer them.