We're not London, but did 7+ selective 2 years ago with a bouncy, sporty (generally) enthusiastic DD.
We did very little, but made DD do a couple of tests from the Bond Books at 8-9 level (with a bit of guidance to make sure she understood what was required from the qns) so she wasn't too phased on the assessment morning.
That said, it seems that she is great at creative writing (which might be an issue for boys) and expressive reading. I think the school did standardised tests when assessing (so they would be expecting better scores from the older cohort).
Looking back (and trying to make sense of what I've heard from subsequent mums), I reckon that about 50% of her "assessment morning" involved tests. The rest was about observing how she reacted to challenges, played with other children, looked "engaged" and interacted with the teachers. And, yes, the teachers were very conscious that the kids were out of their "comfort zone".
We didn't make a big thing of the assessment (it wasn't a disaster if she didn't get in). I can't begin to imagine what even 1 hour a day "targeted revision" would have done to her stress levels after a busy Autumn term.
Assuming that you and immediate family/friends are intelligent souls, I recommend that you chill and talk (and listen) using a rich vocabulary and make sure your child reads and talks with appropriate emphasis.
For some schools, though, money probably talks.
Good luck and keep sane.