Our Beko tumbler dryer is B rated and only cost £180, so that's not too bad. There is a big issue with drying on a radiator or on airers indoors though and that's the moisture. If you've ever had a condenser dryer, you'll know how much moisture is actually in clothes that have been subject to a pretty vigorous spin cycle, and it's not a trivial amount.

Unless your house is well ventilated, and in winter if you've got the heating on, it won't be, all that moisture (around a litre per average sized wash) has to go somewhere. It goes in to the carpet or gets absorbed by the wallpaper...