Trumper's has always a bit this way, particularly in the Curzon Street shop. I think, to be fair, it's easy to write it off as rudeness when in fact it is more of a traditional, old-fashioned approach to retailing - a totally unique British style. I can understand why foreign visitors may find it odd.
The other shop (off Jermyn Street) I find to be less formal, but equally less authentic.
In response to the comment about keeping the product behind the counter/glass - this is because the display units in the Curzon Street store are original. Again, it is to respect the authenticity and history of the shop.