We have 3 children, 2 at SS's. I would not start an formal 11+ work until they hit year 5. They need to read widely and have a very good grasp on timetables. One year spent doing prep. is more than enough. If they have the ability they will pass. Also they need to be taught maths ect in school first as you may teach it incorrectly.
Probably very good advice to stop children getting fed up with it all - but these things are not set in stone. What is formal 11+ work as opposed to informal? And what if school never teaches the maths, or they don't teach it correctly?
Fast mental maths is always useful whatever the test - addition and subtraction facts, multiplication and division facts, quick conversions from cm to metres, litres to ml etc etc, mental maths tricks and shortcuts. If the test turns out to be one that involves high speed but not a huge amount of thought then this will be beneficial as there won't be time to write down much or any in the way of workings. And if it turns out to be one that involves more creative thinking in maths it is still useful to have unburdened the brain from the mental arithmetic load.
My year 5 DD is good conceptually, but probably a lot slower at mental arithmetic than is desirable at this stage. If I had year 4 again I would probably aim at doing lots of stuff completely mentally and build speed somehow or other (haven't a clue how).
Reading well is always going to be useful in any test, understanding maths vocabulary, building up receptive vocabulary - all useful.
The Kent test is still going to be multiple choice, and the writing (if we still have it) would still only be looked at in the case of a marginal fail. So you can pass the test and not be at all great at school-type KS2 writing.
My guess is we end up with a C E M test like Bucks and Bexley (read those fora) or a GL assessment test like our current one from but with some chunks cut out and some others added in, and each of the two papers containing more than one subject discipline rather than as now a pure NVR paper, a maths paper, a VR paper.