Hi, John.
It's been a while since I used Q2000 (using Q2006 now), and I've never used the UK version, but I think the basic steps are the same for all versions. You don't really want to combine the files; you want to combine data from the files, and the Export/Import process does this well, using QIF files to move the data.
After making sure you have adequate backups, of course, just in case...
Open your first QDF file and click File | Export | QIF file. From the next window, choose WHAT you want to export. You probably want to choose Transactions. You may want to choose the Account and Category Lists, too, especially if those have changed over the years.
Then Open your current file and click File | Import | QIF file and let it import transactions, etc. Test the results and, if you are comfy with them, backup your new current file. Rinse and Repeat until all your files are in the current version. ;