You can pre-order the Polar M430 at Power Meter City (PMC) in the USA. Josh made a reasonable-sized order of the white and gray ones and they should be shipping to you towards the end of May.
I’m not sure where the official pre-order channel is in UK/Europe, I’ll update this when I know.
There is 10% off most stuff at Power Meter City with the coupon / discount / promo code: 5KNOV16. HOWEVER It will not work for many new Garmin products. Sorry. That will be at list-price everywhere for some time, plus Garmin sternly frown upon discounting in any case while products are under MAP.
I will also update this post with links to Amazon and with links in the UK as they surface. It looks like Garmin may have been bumped into an early release as too much information was leaking out before the official date. Maybe.
UK Buyers note: Polar M430 and M460 are NOT yet available for UK pre-orders through Amazon. Excuse the image for now, I will change that and the links once it gets listed 🙁