For SEPA (Single European Payment Area) direct debits, you have 8 weeks to get a full and immediate refund. I'd assume that holds for the UK as well.

Many companies (and individuals) in Europe publish their account numbers on their letter head and website, it really isn't a big deal.

Anything else seems security by obscurity.

For SEPA-DD, 8 weeks is for no questions asked refund; in general for non-authorised payments you have 13 months to request a refund, but if it's 8+ weeks they can verify the lack of direct debit mandate before hand - but it seems to be the policy of most banks that they'll refund anyway immediately and let the merchant handle the problems.

So what? Someone set up a direct debit, he can just cancel it and get the money back. Of course it will take a bit (a few seconds with online banking nowadays) but you wouldn't lose any money. There's no way someone can get money from a UK bank account by just knowing the account number, assuming that you check your account regularly.

Not sure about UK but in Poland direct debit is something you need to manually enable and pay small fee for it.

And even if you enable it someone needs to forge your signature under direct debit order to allow someone to charge you.

So still no.