I don't see anything peculiar about Bronte's opinion. The general rule is that if it's merely a replacement, it's an expense, and if it's an improvement, it's capital.
Boilers tend to be rather functional pieces of equipment and unless your new one has major tangible benefits compared to your old one, it's usually far more likely to be a cost item rather than a capital expenditure than enhances the value of the property.
(By the way, I wouldn't be 100% sure that a boiler can qualify for wear & tear capital allowances on furniture and fittings, as I would see it as a fixture rather than as a fitting. - but I think the point is rather moot here.)