I am a Worcester Bosch and Vaillant accredited installers and offer these best in the market brands the preferred options. These quality brands offer long 7 or 10 year warranties, I feel these top manufactures offer my customer peace of mind.
I am also a Viessmann and Intergas accredited installers offering these makes where the installation dictates.
When price vs quality needs a different balance I can offer Glow Worm & Vokera boilers.