> If you're really lucky and you're top of your field you can hit assistant professor by 30-35 in the UK.

This role doesn't really exist on the UK, the equivalent is that of Lecturer/Senior Lecturer. I think you're overstating the barrier to entry -- it's not that high unless you're fixed on a top university. In CS,I know lots of people younger than 35 with SL jobs. Getting to the level or prof after that is another matter but on the whole I think that should be hard.