Evidence suggests that commercial curry pastes are commonly used by BIRs.
I think it may account significantly (but not totally) for what many people perceive to be the general demise in the quality of curries today, compared to those of yesteryear (i.e. the 70s and 80s), when commercial curry pastes were far less commonly available or, therefore, used.
I'd also suggest that Indian restaurants in other countries, such as Australia, predominantly use commercial curry pastes. It is strikingly obvious from the tarty taste (from the preservatives, stabilisers, etc) of their curries. I believe this is why their curries are so markedly different from decent BIRs curry, and particularly different from those BIR curries from the 70s and 80s. Edible and enjoyable, for sure, but not quite British, having neither "the taste", "the smell", nor "the appearance of a BIR curry

I often use small amounts of commercial curry paste, but generally in addition to other spices for "balance" of flavour, much like I often use both fresh and bottled garlic and ginger.