GRP
Filing a flight plan serves two purposes. 1) If you are in remote areas, it helps the Search And Rescue to find you. 2) It tells ATC where you want to go, and allows them to disseminate the information to all relevant units, ahead of you.
ATC is generally very accommodating (honest!) and will accept routes changes, and will notify downstream sectors and units of any changes.
In the case of VFR flights, where the en-route ATC service is mainly FIS, there is very little for ATC to do. Also, VFR flights are more subject to change because of weather conditions. Consquently ATC pays little attention to VFR route descriptions and generally treat VFR flights "as they come".
The real problem in deviating from your route is the SAR aspect. If no-one knows you have deviated and you don't arrive, it makes it harder to find you (assuming a remote area - Birmingham, doesn't count
). If you deviate from the route, but notify ATC, there should be a record at the ATC unit and SAR will be able to follow the notified change.