The boiler should really be sized to the house. I use a Mear's calculator to work each kw required for each room, factor in number of external walls, type of floor, add 10% for system delivery losses and approx 1kw per 50 litres of hot water cylinder volume.
Saying that, 11 rads, a 26kw HE boiler would be ample.
I personally would not go for a Firebird. I would go for a Grant Vortex. Much better boiler.