Last time I was on their Vortex Blue familiarisation the 10yr warranty excluded the blast tube and the flame cell, which is v expensive. I don't think it is the 'whole boiler'.
Grant combis have had a poor rep; the reference to the expansion vessel is because the shell was only just strong enough and when the small, very hot previous expansion vessel failed the extra pressure often cracked open the jacket, resulting in a complete write off. Normally just out of warranty. Got one to do now, coincidentally.
Worcester use Winkelmann (Reflex) expansion vessels which are superior, and seem to have a stronger metal jacket. Not perfect, though, in other respects.
Because of the problems we've had with Grant combis I'm not going to rush to recommend their new one, especially the Vortex Blue which is £300 more for no benefit.
We usually specify Worcester Heatslave 2.
And I can't understand the poor flow rate comment, oil combis have better flow rates than most gas combis. Sure, an unvented would be better still, but I'm sensing the customer doesn't want that level of sophistication.