primary I believe as long as you are an employee you qualify for maternity leave. As you say, you wouldn't though qualify for smp.
I'd probably not say anything. You should be able to dress for interview at that stage so they can't visibly see you are pregnant.
Personally, I would tell them after you have received an offer (when you accept). That way you've been open before you start and will be covered for health and safety etc etc.
I'd probably be careful to avoid applying for jobs that regularly get altered for h&s reasons during pregnancy, e.g. where you're expected to carry anything, where you have to stand your entire shift, where you do extreme late or early shift times.
Depending on your skills I'd probably try and do something high value low frequency. Could you do supply teaching? or office temperature work?
Good luck with the job hunt!