As far as I can tell there is no useful information available. If you have some then please share.
On one of these threads somewhere I did an analysis based upon number of fatalities by cause of death using some figures from a report from a couple of years ago.
Came to the conclusion that the number of non tresspass electrocutions was a handful a year and we have no way of telling what system caused them, or if they were caused by traction supplies at all.
The UK rail industry hasn't done a significant third rail scheme for even longer than it hadn't done a significant OLE scheme before the recent ones.
Until the recent ones surely they occured only a couple of years apart?
If we exclude things like ELL extension which doesn't really count anyway the last one would be the North Downs partial electrification in 1993 wouldn't it?
And the last significant OLE electrification would be the 90s BR projects on the Cross-City or the GWML Suburbans.
Whilst I agree that the risk of cost-overrun is rather high due to lack of experience I believe that a smallish low-cost, low-complexity scheme should be carried forward to find out just what the cost base is in the modern railway.
For example a Bidston-Neeston electrification sized appropriately for the probable service or a scheme between Ashford and Ore.
Such a scheme provides a useful connection if completed but at the same time is unlikely to break the bank even if it overruns enormously.