Optimum working temperature
The optimum working temperature for an office is generally recommended to be around 21-23°C. In order to avoid extreme differences between outdoor and indoor temperatures its sometimes suggested that this can be increased slightly if it's particularly hot outside.
Outdoor working temperature regulations
There's obviously no way you can control the working temperature for working outside, but the Health & Safety Executive provides guidance on working safetly in hot or cold environments as well as how to take steps to protect workers from sun exposure.