1. Many people will only use a CC online for the additional protection it allows.
2. Many people put all their spending on CC for the various point / bonus schemes they have
3. This limits you to UK people only. Surely the purpose for most people creating a web app is to reach a global audience
1) It's little known but the protection under DD are arguably stronger than those under cc (http://www.bacs.co.uk/bacs/businesses/directdebit/collecting...); but your point still holds for as long as this remains little known ;)
2) Fair point, although this varies a lot by country (consider CC usage in Germany) and is not the case for B2B transactions
3) No it doesn't. There's a pan-European ACH network (SEPA DD), also if you look at PayPal they offer Bank transfer in US & Europe using ACH in a similar way