In order to get add-ons simply add their value to your pledge. You will be able to select your add-ons when the PM is open.
Johny Maino minis for example will be available for free for all backers who have pledged 60 GBP or more (of course if we reach the goal). It will be also available in addon section for backers who wants more than one Johny miniature.
When unlocked the SAR Angels is free for everybody with a pledge of GBP 60 or more. It will also be available as an add-on.