This site rather misleadingly (in my opinion) suggests that a rasher of bacon has only 64kcal:
http://www.weightlossresources.co.uk/calories/ calorie_counter/chicken_meat.htm
But that's based upon the assumption that the rasher only weighs 25g. That seemed light to me, so I've just reached into my freezer and weighed some bacon. I store my bacon in clingfilm packages of 3 rashers each. Each pack of 3 rashers weighs just under 150g, which means that a typical rasher of bacon weighs around twice as much as that site suggests. So (trusting the 'kcal/100g' figure from that site), the bacon contributes around 380kcal.
That site also suggests that a (50g) white bread roll contributes 140kcal. Again, the weight given seems rather light to me. (I've not got any bread rolls to hand but I've weighed some sliced bread to give me any idea of the likely weight of a roll). I'd suggest a roll is more likely to weigh around 60g, and so contribute about 170kcal.
So, based on those figures, I'd estimate around
550kcal for your bacon buttie.
Chris