Going by an interview/story written by Pat Gilberts for "Record Collector" magazine (sorry, no date) we have evidence for the year 1987 as being the date for the creation of Lush. I'll pick up the interview where the 'date of somewhat significance' appears:
... Miki was adding primitive guitar to rockabilly garage band the Bugs, and even accompanied them on a European tour to promote their August 1987 album, 'Darkside' (which she didn't actually play on). Around this time, Miki and Emma began hatching a plan for their own band, initially called the Baby Machines. Recruiting two of Miki's classmates, singer Meriel Barham and Kendal-born drummer Chris Acland (then Miki's paramour), they began rehearsing in Miki's bedroom. After a few weeks, they realised they needed a proper bassist, and approached another student, Steve Rippon, who was five years older than the others and originally from Bracknell. 'I was in their class as well,' recalls Steve, phoning from Ireland. 'I was sitting in the college canteen one day, and they sidled up to me, really embarrassed. They said can you play bass? I said, no. They said, great, can you learn in six weeks?
'I went down to an audition at this place behind the Holloway Road,' he continues. 'They were possibly the worst group I'd ever seen in my life. It was an absolute cacophony. The songs were really simple and had titles like 'Female Hybrid', 'See-Saw' and 'He's A Bastard'. I thought, yeah, I can play bass to this.'
The urgency to recruit Steve was because of an impending gig at the Camden Falcon on 6th March 1988, supporting the Rosehips. In the event, the night passed as smoothly as could be expected, and the band branched out to play other venues on the London toilet circuit. They even supported Ted Harris from 'Playschool' at Ealing College. After they cut a four-song demo, internal tensions bubbled over and Meriel was given the push. 'She just wasn't interested,' sneers Emma. 'We'd organise all these gigs and she couldn't play them because her boyfriend was going away the next day or whatever. He came first. But she also felt uncomfortable playing guitar.' Meriel went on to join fellow 4AD shoe-gazers, the Pale Saints.
You know the rest of it. Miki took over the lead vocals position vacated by Meriel when no replacement could be found. And the rest is music history.
You can read the entire article reproduced here: http://www.curve.demon.co.uk/lush/press/p_art3.html