As far as I know, the only people in the UK who have tenure are those who have been in post a very long time... I don't think it's a viable part of university life here, any more....
In a perfect world issues of job/retirement security are supposed to be resolved by academic tenure which, at least in the US, traditionally amounts to a quite livable retirement pension as well. Tenure seems to be an endangered institution, however, and it very much remains to be seen how these things will shake out in the coming years.
As for publishing popular works alongside academic ones, this can be actually be harmful to one's career in some academic fields, insofar as it's sometimes looked down upon.
- N