If it is anything like my build, it will be a challenge, but rewarding :)

I think if you a 1100mm axis, you will want to use 2x ballscrews on it, one next to each linear rail. I guess you are thinking of putting one ballscrew in the middle, but this can cause the gantry to wobble side to side since it can resonance and flex like a trapezoid (since you can't brace the gantry in all directions), so it is better to hold it at both ends.

Where in London are you? I'm near Lewisham.