We live next to a swamp. I know we're not exactly in the same area, but I don't find that the swamp attracts mice or rats. It attracts voles, bats, and small common snakes, but those are harmless. It also attracts mosquitoes, but if the winters get cold enough it doesn't get too bad. Really what you should be looking for is house access points-- little tunnels or gaps where snakes or small rodents could enter the house or have entered the house before. You could hire a pest control person to come out and look at it before you close if it's really going to bother you, because unless the system is different in the UK, the home inspector is going to look for signs of current little visitors rather than non-obvious ways future visitors could drop in. Our little vole system accidentally make a tunnel that entered into our basement area and it became a bit of a vole turnpike one spring and it made way for a couple snakes to get confused and stuck in our basement so we dug a very small trench around the house and packed it with peat stones, then went around and filled every possible crack with expanding foam and touched up with paint. We haven't had any squatters in the 3 years since.