This is flawed from the start.

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

As a heads up I'm from GC but as quick responses:

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

True, there are limits - but that doesn't mean it's flawed. 1. DD also provides protection. Admittedly not at the same level, but the banks do guarantee the scheme against misuse. 2. Very true - but if only DD is offered as an ongoing payment mechanism then many people will use it. 3. The UK is a perfectly valid market to target - and hosted DD provision is an excellent innovation to offer.