Busymum, I express every morning and put bottle into a bottle bag if we're heading out, otherwise it goes in the freezer. If its been about an hour and half-two hours then I will reheat, either at a cafe (asking for jug of hot water), but I also have a small stainless steel jug with me and just fill it up wherever I can (baby rooms at mall etc).
Fresh EBM can keep at room temperature for between 6-10 hours apparently, although I have never tried. But if you were keeping in a bottle bag I'm sure it'll be fine