You can take official Driving Standards Agency (DSA) practice theory tests online free of charge here.
Taking your theory test in the UK
If you pass one part and fail another you will fail the whole test, and you will need to take both parts again.
The questions in the multiple-choice section will depend on the sort of vehicle you are hoping to get a licence for. For example, car theory test will contain questions that don't appear in any other test.
For the hazard perception test there are no separate versions.