If the UK had an MMP system with a 5% threshold the seat allocations would be:
Lib Dem 149
Plaid Cymru 4
Which would mean that a coalition of any two of the three largest parties would have been able to form a government. This would not look like much of a victory for Blair.
(This doesn't predict how people would actually vote under PR - a few smaller parties (Greens, UKIP for instance) would get 5% - the factions in the larger parties would probably be externalised as well).