In most cases The kW output of your new boiler should be chosen depending on the household's hot water needs and not the number of bedrooms or size of the property. The 35-42kW range are designed for larger properties with the need for multiple hot water outlets to be supplied at the same.
35kW boiler's normally produces around 15 litres per minuite, this size boiler requires good water pressure and a little tweeking will just about provide enough hotwater for 2 showers at the same time. 42kW boiler's will give much more hot water flow rate, around 18 litres per minute. You will also find that these larger boilers generally come with 7 to 10 years warranty and have the most energy efficient components such as varible speed pumps and modulating Gas Valves.
On the downside these boilers often need a larger 28mm copper gas feed in order to run correctly. In order to achieve the high amounts of hot water the incoming cold mains to the property must be sufficient. There is no point in getting a higher sized boiler if the incoming cold water delivers less that the hot water output of the boiler.