[Avistew]

Er, no. If he had said "sounded like", then sure "proper British", but otherwise, "sounded properly British" is correct. You need to use an adverb there, not an adjective.

EDIT: here, I'll give a better explanation. Hey, let's say a proper explanation :P.

When you say "it sounds like British" or "it is British", British is a noun, and refers to the language. To qualify a noun you use an adjective, so you say "proper British".
When you say "it sounds British", however, British is an adjective, and you don't qualify and adjective with another adjective but with an adverb, hence the use of "properly".