Is there a publicly available table of annual avarage UK spot market
(wholesale) electricity prices from 1990 to present? I have a hard
time finding any information. My preference is a table of "year" and
"price" in nominal or real British Pound Sterling/MWh.
Thanks.

When electricity is produced, there is less than a second to use it.
The price depends more on the time of day, the date, and the
availablity of a power line to get the electricity to the buyer. A
million more or less separate purchases likely occur some hours in the
UK, many of them lasting less than one second. Sorry, I don't know the
details of money transfer.    Neil