Hi Donna, I have never seen high viz body warmers, but I have had to order PPE for the lads offshore. Could you order warm high viz jackets? Or do they have to be sleeveless for some reason?

Before buying any such equipment you should trial various types in similar circumstances after spending some years working in cold climates with cold winds/rain-sleet etc. I found that its layers with an outer layer that stops wind etc. not thickness that counts as day to day body warmers do not work without some sort of water/wind resistant top layer and a cheap, simple and thin outside layer of plastic with appropriate layers underneath is much better than getting a fewer number of thicker layers as its mainly the wind chill that needs to be stopped along with water getting in and plastic [or similar] does that job quite well - also it depends on what U are doing as if heat is involved e.g. Welding etc. then the plastic is obviously no good at that point in time - also U need to consider sweat etc. getting out and the need to use the loo and its the lower back that is the main place to keep warm noting that many body warmers do not cover the lower back Just some thoughts

As per Bob's suggestion. The solution is to wear suitable layers of insulation under the coveralls. Primaloft is good for mid-layers as it has low bulk, also merino wool undergarments. Also consider wind-proof mid layers. If you go for hi-tech outer garments, they will be more difficult to launder and unlikely to be fire resistant.