I agree with hm -- us about the Oxford transcription which suggests that 'excellence' is pronounced entirely as in English: very strange.
And Oxford's own audio pronunciation doesn't agree with its transcription - as SP (UK) has noted, in their 'UK' recordings Oxford and Cambridge both have the same pronunciation, with the nasal vowel.
Cambridge too is inconsistent: its transcription places the main stress on the final syllable, while, in my opinion, its UK recording places it on the 'ex-', with a secondary stress on '-lence'.
While I too would have said that I agree with Martin--cal in #1, I think that in Britain we do in fact often stress the 'ex-'.
I wonder if, as a general rule, the audio recordings on these dictionary sites are more up-to-date than the transcriptions.