Didn't try it myself. But out of interest what waypoints/routing was in field 15 on the plans that were rejected?
The language of FPLs is very specific that needs radio aids, radial/distances, or lat/longs (not VRPs, town names, etc).
Not wanting to teach people to suck eggs, but a simplistic VFR FPL route example would be flying from Earls Colne to Le Touquet. Field 15 won't allow 'DCT SHEERNESS DCT DVR DCT CAP-GRIS-NEZ DCT'.
It would have to be 'DCT DVR DCT' or 'DCT DET035008 DCT DVR' or 'DCT N513700E0003500 DCT DVR'.
Field 18 would then contain something like: 'RMK/Actual VRP routing: Sheerness VRP dct Dover dct Cap-Gris-Nez dct'.
Is it allowable to file a flightplan from a nearby airfield 4 miles away with a 4 letter identifier rather than having to enter the GPS co-ordinates of our grass strip?
This is possible, however it would be better to enter the strip in Field 15 as ZZZZ, then in Field 18 'RMK/Departure will be from a grass strip at......'
Also ICAO 4 letter identifiers can be used only for Dep/Dest/Altn. If you plan to fly overhead an airfield, an aid identifier would have to be used, or radial, or lat/long.