Speaking as a native speaker of American English, I notice from my perspective that Cantonese speakers tend to sound more "British" than Mandarin speakers. I'm not just referring to Hong Kong, but wherever Cantonese is spoken. Can anyone verify this and why it might be so? In particular, vowels in words like 再，十，去，你 and 想 come to mind.
Probably because HK was a British colony. I did notice that HK Cantonese sounds differently compared to Cantonese in Guangdong, China. Personally I only feel it's softer but I don't know if it is related to the fact HongKonger was influenced by British English accent. Maybe you can do more research and provide clearer examples in your questions.
Mandarin has a strong R sound like American English “car”. But Cantonese doesn’t have an R sound. The Cantonese vowel eoi sounds very similar to British English long O. But neither Mandarin Chinese nor American English have this vowel sound. Many British English dialects use glottal stops in place of final T, just as Cantonese has glottal stop consonants K, T, P. Along with the many English loanwords more likely based on British pronunciation, these phonetic features may make Cantonese feel “vaguely British,” at least more than Mandarin...