So taking a shot in the dark here but different countries use different specs for transmission and of CLID, so the case for you may be:
- You have a 1200 that is not compatible to UK standards
- It is compatible to BT standards, but Virgin uses non-BT standards (ETSI that is used in the EU)
- While you have Virgin service, you may have BT wiring which not always compatible with RJ11 units
A BT to RJ-11 coverter may do the job, but it would be good if you could first test the unit on a BT line and see if it works...then you can narrow it down.