Changes:

2-1/25: released around early 1999 (OS disc V12)
This is v2.5 for the Mk2

2-1/81: released March 2002 (OS disc V19)
This is v8.1 for the Mk2

The changes are primarily bug fixes, there will be very little visible difference.
It is definitely worth updating the Mk2 software, as there is a big jump from v2.5 to v8.1
You will have a much more stable navigation system afterwards

As for TMC, to make TMC work you must have the following
A TMC enabled map disc
A TMC enabled radio
The most current OS software :-)

I suggest you do the following:
1. Update the OS to the latest version regardless of any other steps
2. Get a 2nd hand late model map CD for the UK, making sure it is TMC enabled. I believe they sold TMC and non-TMC map discs in the UK