99.92% of the time when I hear the word says, it is pronounced /sez/. That’s how I say it, that’s how everyone I know says it.
But every now and then I’ll hear a British person on screen pronounce it to rhyme with pays, /seız/.
Is this a regional variant in England, or a personal quirk? Or something else?
I presume you mean /sɛz/ with your first pronunciation?
The /se:ɪz/ version is a second option for me as a Southern Scottish native English speaker. Even without the diphthong, just /se:z/ but I’m damned if I know when I would use one and not the other! Possibly to convey emphasis on the word (’he says he’s going to do it’ expressing doubt as to whether something will ever actually happen despite ‘him’ saying it will) but I couldn’t stand in court and swear that.
Mind you, I have been away from daily life in Scotland for a long time now so that further lessens any value you can take from my comments.
But it is familiar as a pronunciation.