So it really is a million dollar Question, after all you must remain in her good books
I built my wife her first Retro ride based on a nice Muddy Fox Alu Sport (Ok not high end) but she could add her little touches and help with the evolution of the build. And i know the saddle is like a Zeppelin but its a Muddy Fox saddle, and is comfy for her and was purchased by the wife (Its her bike, she rides it so she needs to be happy with it) STX-RC groupset nice and light, and all came in under £100 even with the forks purchase.
£400 should get you something special indeed, but i would seek her input and let her become creative, there is a tendancy to build how we like a bike to be, not how our wives, girlfriends and mother-in-laws would really like (Hence the purple on Lauras Muddy Fox, Her idea, not mine, I actually dont like Purple Ano that much)
"Would anyone else rather be doing something other than marching up and down the square????"