So far....
100 - 199 was being used by ancillary vehicles (vans / wagons).
200 - 999 is being used by Minibuses.
1000 - 1999 is being used by ancillary (vans / wagons).
2000 - 2999 was used by ex OK Vehicles, think it might have been used by some Single Deckers too.
3000 - 3999 was Double Decker
4000 - 4999 was Single Decker
5000 - 5999 is being used by Single Decker, however some early 5xxx numbers were used by downgraded coaches.
6000 - 6999 is being used by Double Deckers.
7000 - 7999 is being used by Coaches.
8000 - 8999 is being used by Midibuses or small Single Decker.
9000 - 9999 is being used as a temporary numbering series.

There has been exceptions to the rules where demo or loan vehicles have used or reused past numbers. A trio of Olympians loaned from Oxford Bus Company around 2000 and a pair of Wadham Stringer Darts loaned from Metrobus around the same time used their fleetnumbers from their respective companies since there wasn't any clashing with a vehicle in the fleet at the time. Same went for 4x DAF SB220 - N-C Paladin Single Deckers (M846RCP, M848RCP, P903PWW and P905PWW) and a single DAF SB220LF - N-C Paladin LF single decker (R33GNW) which used their registation numbers.

A lefthand drive DAF SBG220 / Den Oudsten Alliance B89 bendy bus from the fleet of ZuidOoster in the Netherlands was used on the Metrocentre Shuttle and used its ZO fleetnumber of 7771. Seen here in a photo by Durham Bus Man - https://www.flickr.com/photos/16042610@N07/8899462003 . Ironically the ZuidOoster fleetnumbers used the same font as Go-Ahead Northern as shown on sister vehicle 7772 with ZO. https://www.flickr.com/photos/arthur-a/5103958279

The first Dennis Dart operated by the Go-Ahead Group was numbered in the 4xxx range before being number 8000 (H802OPT).

The first batch of MPDs were numbered in the 8xxx series (8186-8200) because they were Darts before they renumbered to 486-500.

Technically because the SPDs were just under 12 metres, along with most of the Excels except 8125-8132, they could have been allocated 4xxx numbers, but were seen as lightweight single deckers compared to the Volvo B10BLEs and DAF SB220LFs that were arriving around the same time, so were thrown with the rest of the Darts in the 8xxx series.

However this idea changes when you look at the Optare Versa, Wright Streetlite and Yutong Electric buses are actually split between fleetnumbers according to their length. Those around 12 metres being numbered in the 5xxx series whilst the shorter vehicles (11 metres below) are numbered 8xxx.