We don't generally advise people to share an OmniFocus database as you describe. The main risk is that if you both edit an action at the same time, there can be data loss. For example, you change an action's date at the same time he changes its context. Once everything's synced up, only one of those changes will still exist.
However, if you would still like to do so, here's how (without much typing). Please read this whole thing through and ask if you have any questions before embarking.
0) Make backups of each of your databases on your respective macs, just in case anything goes awry.
1) Make certain your father's OmniFocus on Mac and iPhone are perfectly in sync. (Sync one, sync the other, sync the first again.)
2) On his mac, File > Export...
Choose somewhere convenient like your desktop.
Set the format to OmniFocus document.
3) Transfer this exported document to your computer by any convenient means. (Email, USB drive, etc.)
4) On your computer, double-click the file. It should open in OmniFocus, side-by-side with your existing database.
5) Drag-n-drop or copy and paste whatever you want to put into your database.
6) On your father's mac, change the sync settings to match your computer's sync settings. When it syncs, there will be an alert "Replace your database and start syncing". Choose "Sync".
7) Repeat step 6 for your father's iPhone. The dialogs are slightly different, the general process the same.
Now you should have the same database in all 4 places.
We do not have a UK phone line, but you are very welcome to email
omnifocus@omnigroup.com or post here if you run into any troubles along the way.