GSAK is supposed to be compatible with both imports and exports of .GGZ files. I am not sure how familiar you are with GSAK but it could offer you a solution. It is excellent when dealing with cast quantities of data. You might be able to import your rather large preloaded .GGZ file from your GPS to a new database in GSAK, then manipulate the data i.e. run a macro to determine which ones are now archived, then another macro to determine, which ones have already been found by you.
http://forums.groundspeak.com/GC/index.php?showtopic=333215
To summarize as I tend to ramble a bit:
"Load the 250k pre-load GGZ into GSAK, cross reference with your own 'found it' database, then update for archived listings, and export back to your GPSr."
http://forums.groundspeak.com/GC/index.php?showtopic=340925
Given the huge numbers of caches in the average preloaded .GGZ file, this option may not be viable. The "refresh cache data" option has a limit of 6000 caches per day (10,000 if the light format is selected). It uses the same api call as "
Get geocaches" and hence is bound by the same limits.
http://gsak.net/help/hs46010.htm
Eventually, you would be able to export this newly updated, slightly smaller .GGZ to your GPS.
Another option, which is so much easier and less time consuming is that you run different pocket queries for your area and any area you are likely to cache in your part of the UK, then import them all to the same database in GSAK, run minor macros to update critical data on an ongoing basis, add newly published caches, remove duplicate caches, update logs, update coordinates, etc. Every time you are going geocaching, you simply export this updated database as a GGZ file to your GPS. It sounds complicated but it is what a lot of geocachers do in my area. It works for them.
The preloaded .GGZ file contains only a select sample of caches from all over the world. When I looked at the version that was on my newly bought Garmin Montana 680T (date of preloaded GGZ creation: September 9, 2015), I noticed that it was only pre-programmed with cache information about a couple of caches in the Bay Area. Most of the data was very stale and too old to use or rely on. One or two of my older geocaches were even in the database. I had archived them last year and chuckled when I saw them on the screen. The preloaded GGZ file on certain new Garmins is a great idea but I think that it is only meant to get people more interested in geocaching. For many who are very new to the sport, the idea of the complicated process of running a pocket query or downloading geocaches straight to a GPS through a USB cable might be a bit daunting. I know that it was for me initially. At the time, I did not want to damage my GPS and found the user interface and terminology a bit too technical.
You could try reaching out for help from your local geocaching association/club or go to a few events where I am sure some geocachers who have faced the same issue will be there and be happy to share their knowledge and experiences.
I hope that this helps.