can anyone tell me why a probate would be 12 years after death
As you say probate, rather than administration, there was a will & for that to be put into effect probate was required. Reasons for the delay could be as simple as that the will only came to light then (perhaps left for safe-keeping with someone who wasn't aware he'd died).
Otherwise I'd guess that perhaps the will was written between the accident & his death & that perhaps his widow wasn't aware that formal probate was needed until some much later event. Perhaps he had an interest in some land/property that was being sold/transferred c1870 such that it was then realised that the legal formalities had not been gone through & were now necessary.
The under £450 figure - these values vary at different times, but seem usually to be below limits for certain taxes that would be due at the time of death.