Remember that due to the strange map projection used in the UK Cornwall is actually 2 hours further away from anywhere else in the UK than you think.
York to Didcot is a full day drive, it would take 3.5 hours WITHOUT TRAFFIC. There's no such thing as 'without traffic' in the UK unless you want to set off at 2am.
Last time we drove from Heathrow to Manchester and back for a holiday. it took 3 hours at 11PM to get to Manchester, it took from 9am to 6pm to get back and we didn't get over 50mph on any of the motorways we were on.
Also remember its not the US. You could spend a month in London and still not have seen everything. York needs at least 3 days,probably more, most visitor attractions need at least half a day and probably more, and there are generally lots of things to do almost anywhere you stay.
DON'T return you car to Didcot, assuming you're using a major car rental company, drop it off in York and get a train to London, or wherever.
In York you have the Castle museum, the National Rail Museum, Jorvik, the Minster, Fairfax House (ala Fairfax County Virginia), and don't forget tea at Betty's which will take 15-30 minutes standing in line but it's worth it.
As others have said you're trying to see too much in too small a timeframe, Shrink the number of destinations and spend more time in each one.
Regards,
John P