@John Smith 19
Is there a sentence missing from your post? Here goes anyway.
The UK enrichment plant you're thinking of may be URENCO's Capenhurst plant, near Chester. At one time there was a selection of nuclear-related sites in and around Warrington and north Cheshire, not sure how many are still active.
Wrt processors and languages: in the days of ladder logic, you might well get a proprietary processor effectively built from discrete logic. That was a long time back. Then along came processors based on off the shelf chips such as the AMD2900 bitslice, or based on off the shelf computer innards such as the PDP8 (honest [1]).
Siemens S5 and S7 PLCs and indeed most of the recent non-bottom-end market can be programmed in a variety of languages besides ladder logic.
One of the nicer, "open standard", languages was called FunctionChart/FunctionPlan (it's an IEC standard). Whatever the source language, the development tools on the host would convert the user input into data to be downloaded to the PLC, data which in turn gets executed by the processor on the PLC. Right now I don't know what processors are in common use or whether the PLC has an OS of any significance.
"This is a *lot* of trouble to disrupt (or destroy) someone's *very* specific set of industrial plant."
Quite. Almost, but not quite, an unbelievable amount of trouble.
[1] http://www.plcs.net/chapters/history2.htm