As @muggles correctly says you can not install these without a G3 qualification.Be aware that fitting such an appliance is not a DIY job - it must be done by a G3 Registered installer and is notifiable to Building Control
McDonald Cubeflow have the wall mounted rectangular heaters with built in header tank
https://www.mcdonaldwaterstorage.com/products/rectangular-hot-water-tanks#CubeflowMini