Pre-test can mean a number of things.
The thing that is usually commonly meant is that it is a test sat in Yr6 for offers for places at the start of Yr 9.
The test might be internally set by the school. It might involve VR and NVR on paper or electronically and it might involve writing and maths too. It might be the test set by ISEB which can be electronic or paper based and might be sat in the Prep school or in the senior. It might be the only test sat with offers following, or used to reduce the number of people qualified to sit a later stage exam. It might be used to give conditional offers which then require a certain Common Entrnace mark at 13 (which no-one in practice fails to achieve).
Essentially, pupils are tested at 10/11 these days whether they transfer at 11 or 13. In this sense it is all 11+.
In terms of standards, to be honest it's not the difficulty of the test that is relevant so much as the mark required to get an offer. Many seniors use the ISEB pre test, but the more selective schools which have more and higher quality applicants can require a higher mark before giving offers than the less selective and popular schools. The required mark will vary a little every year.
So, the thing to make sure you are clear on is the format of the exam for any schools your child will sit. Some independents use CEM and some GL. Some use paper format and others electronic. Smoke test VR and NVR or just one of those and some require a story and a comprehension and some a maths paper. You need to contact the schools themselves and clarify the requirements and MN cannot give accurate or up to date info on every individual school - it is vital to prep for the schools you will sit. It is also useful to have a sense of which are 'easier' or 'harder' in terms of standard required in the exam and any people advocate having an aspirational school and a more definite one.
In terms of difficulty compared to the state school 11+, again it will be the standard to get an offer rather than just the papers themselves that differ. Wholly selctice counties such a s Bucks which take 1/3 of cohort will require a lower standard for a pass mark. The superselective state Grammars which admit purely on score and can have over 2000 applicants for just over 100 places are probably harder to get admission to than pretty much all independent schools, because the sheer numbers mean the standard required to get an offer is just so high. Every year, some people get offers from top Lomdon schools and miss out on QE boys and HB girls. It can happen the other way too, but seems to less regularly and often those with the most selective state school offers get scholarship offers from top independents too.