VM Mobile seem to operate as a completely different business to VM broadband and TV most of the time, so different rules apply (hence two credit checks), but also worth bearing in mind that the credit risk can vary depending on what you're ordering. The credit risk on a TV and broadband deal is a lot lower than that on a mobile deal if the mobile deal includes a handset. Added to that, if you've just signed a fixed line contract, that's used up some of the unsecured credit "lives" you have. There's many gross simplifications in that, but what it boils down to is they treat mobile and fixed line differently, and there's no right of appeal.
Other mobile operators (even O2) have different rules, and might offer you a deal, and in that case you'd also qualify for various Volt benefits that you don't get if you join VM Mobile. I really don't see much customer appeal for VM Mobile at the moment, and I'm confident that is entirely deliberate.