This would have been a great addition to the diesel legends pack! (Awaits someone to tell me the 55 never ran on the GWE )
They did do Trans-Pennine workings towards the end so it's fairly plausible as there's plenty to record and could layer in on NTP.
The Southern Region even today are edgy about certain locos operating on their patch ! The class 40 was a deffo NO NO! I'm not sure about the Detics, But im sure the HST sets had to be modified, with regards the trailers and their swing. The deltics weight wise might be forbitten but they had the same bogies as the 37s so im un sure.
The Deltics are bulky but surprisingly light especially for a 3,300HP loco'- just shy of 100 tons IIRC or just over 100 tonnes.
Deltics or BR Class 55 locomotives are cleared for running on the Southern network, but they never did because that kind of power wasn't needed or required. The Class 55 is RA5 The Class 66 is RA7 (so in theory a Class 66 is bigger), the Class 66's are cleared to run on all Kent/Southern routes. The Class 55 weighs 101 tonnes. The Class 66 weighs either 126 or 127 tonnes depending on the sub class. So the Class 55 could and may one day run in Kent/Sussex, it's not banned.
D9000 used to run in Kent to Ramsgate from Birmingham on “Summer Saturdays” around 1999/2000, around 18 years after they finished in service, a great day out on a service train! The track clearances may have changed in the 20 years since though