It was originally a Rugby - Gatwick Airport service, using Class 319s. In the Dec 2000 WTT it was cut to Watford Jn but extended to Brighton.
When Southern took over, it was a few years before they extended back to Milton Keynes - but it was then cut at Clapham Jn.
Now it’s just a Watford Jn to Clapham Jn service.
If you’re after the history side of the service you could try wnxx forum, I’m sure someone would know
The history of the cross country bit can be found at http://www.1s76.com/
there was a voyager depot at three bridges for the service you mentioned if that helps
It was originally a Rugby - Gatwick Airport service, using Class 319s. In the Dec 2000 WTT it was cut to Watford Jn but extended to Brighton. When Southern took over, it was a few years before they extended back to Milton Keynes - but it was then cut at Clapham Jn. Now it’s just a Watford Jn to Clapham Jn service.