The subject of overspeed issues with digital tachographs due to the UK having a higher speed limit than the rest of Europe has been raised on here before.
There have been reports of drivers having been in the UK and incurring overspeed warnings although adhering to the UK speed limit and then having their wallets lightened a few weeks later by the gendarmerie.
I may have a solution to this problem:
You do not need to remove your card from the digitacho to do this!!
When boarding the boat/train, go into the menu and select: entry driver.
“end country” will now be displayed.
Scroll through to the appropriate country (B,F,NL, etc)
Press OK.
This end country is now stored on your digicard with date, time and km reading.
Upon arrival in the UK, go into the menu and select: entry driver.
“begin country” will now be displayed. (If not, use the arrow key to scroll)
Scroll through to “UK”
Press OK.
Your entry date and time into the UK, along with km reading, is now stored on your card.
The time difference between the two inputs will be consistent with the duration of the ferry/train crossing, whilst the km reading will not have altered.
Upon leaving the UK do the same procedure in reverse.
So, when an overspeed reading is generated and stored on your card as you sail down Windy Hill, this can be shown to have occurred in the UK, where a higher national speed limit for HGVs applies and is therefore “Out of Scope” (to use digitacho speak 
) of this particular operating parameter of the device.
Theoretically, if the overspeed has occurred during the time when the vehicle has been logged as being in the UK, you should be in the clear.
Indeed, the cheese eating surrender monkey will probably kiss your feet whilst begging forgiveness for having wasted your valuable time, before buying you a coffee and waving you on your way with a cheery “Bonne journée!!” (Or not 
)
.