I think you are right about a 5y battery warranty in the UK. However, since you are essentially a running advert for Mitsubishi Electric, and the fact that Mitsubishi could, if they wanted, honor a battery failure past the warranty period if they wanted, perhaps it would be good to write them a letter. May also be good to visit a dealership and butter them up before you do that too.
I assume the car has full Mitsu dealer service history... otherwise you stand less of a good chance.
I run two of them, and would also be interested in a few spare cells, but... not an entire pack - so where I unfortunately cant help, it may be useful to start a thread for a UK buying group to split a pack when one comes up again.
There was one on ebay for £3000, which makes it about £35 a cell... plus all the messing around.
Also... I note that the Outlander PHEV uses LEV40 cells, not LEV50s, which (unless you could retro), does not bode well for Imiev pack rebuilds... as I was going to suggest looking for a breakers PHEV pack.
2012 I-MIEV Keiko Silver 11K
2010 Insight ES-T 43K
2001 Accord Type-V (F23 manual)
2009 Hornet CB600F