To put this into context, do you fly a Cessna 150 or a Boeing 787 ?
The pressure difference will all depend upon the number of Isobars you cross, just like crossing Time-Zones. If you fly a C150 then you are unlikely to cross many Isobars (and even less Time-Zones !)
So looking at todays Bracknell Synoptic Chart...
Bracknell Synoptic Charts 12 - 120 hours | NOTAM Info
Even if you went from the Wash to Cornwall you would only cross 4 Isobars, a difference of 120ft to your altimeter.
Bracknell also give you the Geostrophic Wind Scale, which shows how close together the Isobars need to be to give a certain wind speed. Unless you fly in wind speeds over 40kts, you are not likely to cross many Isobars in a C150.