We currently have a dataset of speed limits in http://roads.data.tfl.gov.uk. This data is quite out of date now and we’ve been unable to source an updated version due to licensing constraints. We are hoping to find a way of overcoming these in the next 6-8 months but in the meantime have taken the difficult decision to remove the data from our open data catalogue. I will post again when I have an update. Thank you for your understanding!