Say you are routing Elstree to Bembridge (assuming it's back in action!), via somewhere near Booker to make the turn south but in any case avoiding LHR class A. All in IMC but out of airways and no GPS just standard VOR/NDB/DME.
In Class G, you can fly anywhere. Just get the map out and draw a route
By all means get a radar service but there will be hardly any GA traffic on an IMC day anyway...