I know a lot of you work in cold environments but I work in an office (graphic design) so end up sitting on my arse pretty much all day. We have no heating apart from a blower which takes some of the sting out of the air, but we can only use this very occasionally as it is not meant to be used for prolonged periods of time. I'm sure there is a law against this.
I feel your pain. The heating in our building broke last year and we were down to 8 degrees for a few days.
We're all developers - so the job is being sat down for 7 hrs, just typing. So there's no real way to continually be warm.
The biggest issue is just cold hands. Typing with cold hands is a PITA and typing with gloves on is impossible. So whilst your body can be warmer with more layers, there's nothing that can be done with the hands!
There is no legal minimum (thank god, I'd have staff walking out on me) - but as posted earlier, a "suggested" minimum from the HSE.
Blower heaters are no real use, they won't increase the temperature in the room very much (only feel hotter if you are in front of it). Instead, we got a few electric oil radiators; a single 3kW oil radiator could heat a 500 sq. ft. room from 8 degs to 22 degs in ~2hrs. So its far cheaper than a blower being on all day, its near silent and works a treat.
We've got the spares stowed for a rainy day; but if its always cold where you are; get an electric oil radiator - they're only about £80 for a half decent branded unit with electronic climate control - and ~30p per hour to run during the initial heat up (2hrs) - then its only pulsing on thereafter. Works out about £1.50 a day in electricity.