Why the effects of flooding in Bangladesh are worse than in the UK?

Answer 1

Bangladesh is one the Ganges delta, so has a major river flowing in to it (and therefore intense rainfall higher up the river means a substantial increase in the volume of water flowing in), and approximately 70% of the country is less 12 metres above sea level, so when the rivers burst their banks the water spreads far and wide.

Added to that Bangladesh is a substantially poorer country, there are fewer flood defenses and buildings are of a less flood-resistant quality (and denser than the UK, it's the 9th most densely populated country in the world, so even a relatively small flooded area impacts on a lot of people).

In the UK, by contrast, it is relatively rare for a flood to hit a major built up area, generally speaking the monitoring systems provide warning in enough time to raise flood defenses and spread sandbags and other protective items across the affected areas to prevent damage to property, and evacuate populations where necessary.

Answer 2

because Bangladesh is closer to the sea or ocean.