I love photographing in The Lakes and fortunately for me it's an area that's on my doorstep, or at least it is when I'm in the UK (read, not often enough).
I'm not sure I can offer you much in the way of suggestions on the Nikon question, but I can perhaps off a bit of a comparison from my move away from my 350D last year. I always knew that I wanted to go the full frame route, so when the opportunity came, I got myself the 5DII and the lenses I needed to upgrade from my EFS stock. Which was a great move, though I perhaps hadn't banked on expecting to struggle so much with the consequent weight gain.
So time passes by and I'm finding I lost something in the move, that being the IS capability and familiarity of working with my 17-55mm f/2.8 EFS. In a moment of weakness, my willpower cracked and I found myself buying a 7D body to use with the 17-55. So now I have a top notch full frame and an equally top notch crop sensor body, and I switch between using them as the situation suits.
So basically what I'm suggesting is that there is no real need, other than purely financial, why you should think that the D90 should be a compromise. If you get that now, there's no reason to think that it will languish in a cupboard should you subsequently decide to get a D700 or it's next incarnation.