If I was running BT (or O2, Virgin, or any of the other bit internet companies that have been pushed into filtering their internet by the UK government), I'd do it very simply.
I'd block everything other than
CBeebies (the BBC Children's channel/site), and declare the rest of the internet as unknown territory, and thus plausibly full of adult content.
If pushed, I'd offer to check additional websites, and add them to the approved list, at a cost of £5 per page, in order to cover necessary expenses for manually testing. Payable each time a page changed, of course - you can't be too careful where children's safety is concerned.
Original post on Dreamwidth - there are
comments there.