Today isn't an option for the form I'm thinking of (tax free childcare); you can't set up payments for the same day. Really you want "asap" but I was simplifying for discussion.

I agree the tax-free childcare form could certainly be improved (I've even written a script to do the three-monthly reconfirmation for me, see my latest gist, though that's policy rather than tech!), but you can set up payments for the same day, you put today's date in (that is what I do; it gives a "You've asked for a payment of £x to be made to [site] today" extra confirmation step). There's nothing in the service manual page that says they couldn't enhance this form with a button as you're suggesting, and hopefully one day they will, though you're always actually worrying it could get worse...