To be fair, Douglas, Halford's AutoCentres are 'proper garages'. They're often located nowhere near a Halford's retail store (and thus have entirely separate managements) and all of their staff are just as well-trained or experienced as, say, those at KwikFit are.
Much as my cynical nature would also get me wondering what the catch was in their offer, I can't see that they're any more likely to try to boost business by failing a 'borderline' car than many independent garages might be. After all, a mechanic at an Halford's won't have his pay directly connected to how much money the firm takes, so he couldn't care less about whether or not a car is passed or failed. An independent guy though might be tempted to fail a 'borderline' vehicle because he can then make more money for himself through fixing it.