You have submitted your UK application to the VFS in Spain and you notice that there's an error in the planned arrival date field and want to correct it...
Am I able to cross it out and write in the new one?
At the stage where you print the application, but before you report to the VFS, you can make a small pen and ink correction to the paper form. Use a single line to cross out the wrong date and enter the correct date above it. It must be indelible ink (no pencil, or eye liner, or whatever -- and yes, I have seen someone use eye liner on a UK application form). Make an ink notation in the 'remarks' area to indicate what you have done.
This works if the correction is straight-forward, like a planned arrival date. If the correction is more complex, then you need to delete the application and make a fresh one. If they receive an application with lot of pen and ink corrections they will reject it (but not refuse it) and return it to you as an invalid application.
If you have already handed your application to the VFS, then there's little to be done except cancel it. For reasons of security and "application integrity" they will never accept a change over email unless it's coming from a member of the UK Law Society who they know, and even then it needs to be a small, straight-forward error.
Note: Applications submitted to a VFS in Spain are sent to Lunar House for processing and decision-making. The turn-around is about 5 - 7 days.