get yourself into paint shop and buy a primer paint in a preasurised can. spray 2 - 3 layers on (wait some 5 - 10 minutes between layers), let it dry (WELL!) and sand down once under watter with waterproof sand paper of 1000 granity. then spray on one more thin layer, lett it dry (WELL!) and sand again under watter with 1200 granity.
btw did you know there are more types of polyester body cement available? rather then buying larger can of basic type buy two of them next time - a basic one and fine finish one. of course, using fine finish one will not save you from using primer.
regarding Bondo... I kinda doubt you could not find Bondo brand in UK if I can buy it in local general store here. in any case the brand is not important. simply ask for polyester filler.