If it's been designed not to need a wet heating system, then you won't need one. If you build to the PH maximum allowable space heating of 15kWh/(m2/a) then you will probably need a conventional wet heating system. We lived in East Kent near the coast and I designed our PH so we didn't need a conventional wet heating system. As long as the house is designed using the PHPP with the correct weather data, the actual required internal temperature, ours was 23C, then it's feasible. We heated ours with three electric towels, one in each bathroom, 4m2 of electric UFH in the kitchen and warm air heating through the MHRV from an EASHP. The warm air heating only came on when the outside temperature was low for some time. Be careful of people who say, for example, we don't need heating, but don't tell you the bedrooms are at 14C and the living room at 18C. We had the whole house at 23C 24 hours a day.