Wiktionary lists two different UK pronunciations of data:
/deɪtə/ (UK, US)
/dɑːtə/ (Australia, UK formal)
Under what kind of circumstances would the /ɑː/ sound be used?
Which pronunciation is considered RP?
Even in formal environments, I hardly ever hear /dɑːtə/ any more. 15 years ago it was, while not common, still in usage in computer halls (back when they were computer halls) but it seemed to vanish along with the move away from mainframes to PC based servers.
The OED gives only /deɪtə/. I've not heard anyone say /dɑːtə/ and if I did I would think it pretentious.
I have heard /dɑːtə/ used occasionally when referring to a collection of numbers or facts in general, and /deɪtə/ almost exclusively when used with computers or data processing. /dɑːtə/ is more common in the U.K., and /deɪtə/ seems to be much more popular in recent years both in the U.S. and U.K.