Sorry Claire, I know they're a huge downside to sharing your home with a cat, but they're definately necessary. If you don't have one you aren't giving her the option of toileting indoors during horrible weather and if she really hates how it looks outside, she WILL just pick somewhere to "go" at random!
My cats are indoor/outdoor cats too, but they have three cat litter trays that I clean out daily. (Two of them use them, two go outside. It's all down to choice!)
EDIT: Don't know what the decor of your house is like, but they do "funky" litter trays now, check these out:
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