The DAF DB250 / Alexander ALX400 combination was mainly designed as a London Bus. Lower geared with a straight staircase, with S-reg and T-reg examples having a centre staircase, whilst V-reg onwards having the staircase behind the drivers cab. All bar 5 were ordered for routes operated by Arriva in London. The 5 that didn't go to Arriva was a single example new to Abus, Bristol and 4 examples to UK North which later operated for Stagecoach (which also meant it operated ALX400 bodied vehicles with all 3 chassis makes.) These 5 V-Regs were also London Spec vehicles albeit single door.

Ironically Arriva NE had operated all 3 types, including both versions of DAF DB250. Here are all the variants....
DAF DB250LF / Alexander ALX400 (London Spec - Centre Staircase) - https://flickr.com/photos/gjm-photogenic/52890269686
DAF DB250LF / Alexander ALX400 (London Spec - Forward Staircase) - https://flickr.com/photos/gjm-photogenic/52890456564
Dennis Trident / Alexander ALX400 (Provincial Spec) - https://flickr.com/photos/gjm-photogenic/51185724925
Volvo B7TL / Alexander ALX400 (Provincial Spec) - https://flickr.com/photos/gjm-photogenic/51194200150
Volvo B7TL / ADL ALX400 (Provincial Spec) - https://flickr.com/photos/gjm-photogenic/52826001995
Volvo B7TL / ADL ALX400 (London Spec - Forward Staircase) - https://flickr.com/photos/gjm-photogenic/52826001275