Hob, some issues to consider. Public liability insurance, vet care sorted out while you are "in loco parentis" so to speak. Can you authorise vet treatment up to a certain level in an emergency? Insurance details? Does taking in such dogs have any impact on your household insurance?
Agreement in advance on liability if dog is lost/goes missing. Agreement in advance as to whether dog comes off lead, and if so, where and how. Who will provide food? Any special feed requirements? Crate? Blankets? Agreement in advance on amount/type of exercise/training.
Local authority licence - in Wales, home boarders are licensed - not sure of exact position in England.
Medication - ensure you have full list, and sufficient supplies to last whole holiday duration.
Children - ensure they are trained to not disturb dog. As they don't have a dog full time in their home, it's really important they understand the behaviour needed and possible consequences if they don't.
Grooming/bathing - will you undertake this while dog is with you? Any special tools/techniques?
If dog is entire, will you guarantee (if female) to return it non-pregnant, if it comes into season while with you? If male, will you cope if it wants to go chasing after any local bitches in season?
Is your garden secure? Have you considered where in the house you willl allow the dog? Stairgate fitted?
Car/travelling - will the dog travel in your car? If so, in what part of the car? Boot or back seat? If back seat, do you have an appropriate safety harness/seat belt? If boot, do you have an appropriate dog guard/crate fitted?
Microchip company - ensure chip company is told of your details while dog is with you for holiday period. Ensure you know local dog wardens contact details and arrangements for strays in your local area.