is £35 a high cost for one of these? It would be 1 per property, sounds alright to me.
For your other questions I think this paragraph suggests they are good to go really (not sure about cleaning, do people clean out bird boxes?):
According to Bourne-Taylor, multibillion profit-making housebuilders have signalled in high-level government meetings they have no objections to the bricks, which are widely made by conventional brick manufacturers. There is already a British Standard for them, which means there’s no government investment required for development, guidance or standardisation.