Hi Activity. Your question is interesting in that the two search terms you presented suggest two different types of business - or perhaps one business offering two types of service.
"Flowers delivered" suggests search results would produce florists specializing in delivery (who knows some of these businesses may not even take walk-in business). "Flowers by post" obviously suggests this business would mail them to you before they wilt and turn brown.
Alan's keyword "florist London" suggests to me I would find a florist in London that I could walk or drive to and buy some flowers on the way home from work.
Perhaps the first question is not whether you can conquer the giants, but what is the business model you or your client intend to pursue?
Once you've identified what kind of business you intend to run, do your keyword research for your niche/approach and build it from there. Giants were babies once too.
Unless a person has a boatload of cash to spend on PPC, then felling the giants or taking some of their business will be challenging in the timeframe you suggest.
Include in your research a little work in Google Insights after you've identified your business model. You may get some good keyword ideas for localizing your SEO at startup.
Finally, check out this thread on creating the perfect website: http://www.seomoz.org/q/seo-list-for-creating-the-perfect-website.