Have in 1603 King James establish his court at York and then have the new british Parliament establish itself there either in 1603 or 1707. Both are so that the new kingdom's capital is geographically neutral and central.
How will that change:
York and the rest of Yorkshire?
London and the South?
British Politics?
The accents of London, York and an ATL Recieved Pronunciation?
The railway network, including High Speed One?