Update

So I went back to Bleach London for a follow-up chat after my disastrous strand test.  I was pretty disappointed that I won't be able to just dye my hair grey and be done with it, but that's life.  Aside from trying to bleach my hair a few times and see what happens, there's not much that they can do aside from bleaching my roots every few weeks, so I end up with some kind of upside-down ombre look.  Once that has grown out and I have cut all of the old dyed ends off, then we would be able to dye my hair grey and assimilate it into the roots.  

I am not sure that I like this idea.  I think that it would look very strange, and wouldn't be all that different to just growing out the grey roots - just a lot more expensive.   I really don't know what to do.  I am not sure that I have the confidence to just grow out the roots, as I don't think it would look very nice.  

I asked about my hairdresser friend's idea to put highlights through it and she said that it was a terrible idea, because if and when we did come to bleach it, we wouldn't be able to get an even colour.