I want a high flow system so I can have a decent shower

- the builder's recommended a "Alpha CB50"

I know nothing about alpha as a company or their products. However, the
specification of the hot water side looks good and should be enough for your
needs, unless you are planning a panel shower or other such monstrosity. It
isn't condensing, though, so has 80% efficiency, rather than 90% efficiency.
That will add over 10% to your fuel bills compared with a condensing type
(but will save at least as much compared to your old back boiler).

Christian,