a friend's husband does this kind of work and he is based in Peckham. I have not used him myself but I can vouch for him as an honest and reliable person who has a background in this kind of work:
Jon - 07985 930 875
I was going to get shelves made once, but then walked past Shelfstore in Finchley and bought my shelves from there. They sell modular shelving systems, but they can all be tailored to fit your particular space.
For the amount of shelving I needed, they worked out very favourably cost-wise compared to having shelves built, and what I like best about them is they can easily be dismantled and taken with you when you move.