According to the guidelines set forth by Kickstarter for the US community,
http://www.kickstarter.com/help/community, promoting a project on other projects' pages is prohibited and considered spam. I am uncertain if the UK version of the guidelines is different. Kickstarter encourages their users, backers and creators, to report violations of the community guidelines by providing a multitude of methods to report perceived transgressions to the rules.
ACLeathercraft, if project creators are directing you to post a link in the comments, then they may be unaware that they are asking that you violate Kickstarter's posted guidelines. I'm not sure how to avoid the problem you experienced, ACLeathercraft, if the project creator will not take the time to post your link themselves. Perhaps willing project creators could try offering a blanket approval on their comment board for backers to post links to other projects in the comments. I don't know how Kickstarter would respond to complaints on a comment board if it fell under blanket creator approval.
The most effective form of cross promotion I have experienced as a backer and a project creator is a positive comment related to both projects, a link to the other project, and a small photo in an update to backers.
(My opinions and experiences are my own and are merely offered as alternative views on the situation being discussed. My post is not intended to offend, merely to participate in the discussion in the hopes that I might be of assistance (however limited) and learn from the experience.)