There are no hard and fast rules for VFR (there is for IFR). One reasonable rule is to add 1000' to the heighest obstacle within 10nm of your track (some use 5nm).
The heighest obstacle is:
* Terrain Height + 300' (because anything under 300' isn't shown on the CAA VFR charts.
* The height above the terrain of any recorded obastacles. The chart shows two figures for each obstacle: one in brackets is the actual height, the one out of brackets is the total height - the one you are usually interested in.
The quick and easy method is to look at the pre-determined Maximum Elevation Figures (MEF) as marked on the chart within each lat/long bounded quadrant. The numbers show thousands and hundreds to the nearest hundred feet and show the heighest obstable altitude (as calculated above) in that rectangle. All you need to do is look along your track, get the biggest number and add 1000' to it (again within 10nm).
*NOTE: The average male thumb is 10nm 1:500,000 chart. Very useful for all sorts of navigation as well as a quick MSA lookup should you need to divert unexpectedly!!
**NOTE: The MEF on the CAA charts require you to add 1000'; some other VFR charts like Jeppesen don't require you to add 1000'.