When discussing the English of places like Singapore and Brunei, we tend to say Singapore English and Brunei English.
In contrast, if we are talking about the English of places like Britain or Japan, we usually say British English and Japanese English.
Why do we use the nouns Singapore and Brunei but the adjectives British and Japanese? Why don't we say Britain English or Japan English? And why do Singaporean English and Bruneian English not sound quite right?
A posting on Language Log (here) suggests that we tend to use the adjective for countries but the noun for states; so we talk about the Canadian Parliament (it's a country) but the California Legislature (it's a state). This would seem to suggest that Singapore and Brunei are being treated as states rather than as countries. Well, maybe. But I suspect it might be more connected with size than status: Singapore and Brunei are rather smaller than Britain or Japan.
One other factor is that it seems increasingly common to refer to the France team and the Spain team rather than the French and Spanish teams, even though they are clearly both countries. Perhaps the use of the noun is winning out. So maybe use of the noun with Singapore English and Brunei English is a sign of modernity and not an indication of lack of respect.