You really need to browse the websites of the big dealers, (Hattons, Rails, Cheltenham, The Signal Box, etc.) they often have a greater range because Hornby only advertise their latest ones and of course their own brands.
Remember that if you have a steam based layout AND you want to stay looking authentic you need to choose the types that co-existed with steam. If in the other hand you are none too bothered with authenticity the world is your mollusc.
Just a few notes in case you want to be authentic -
Main line diesels first appeared in the late 1940s,and early 1950s and then only a handful of experimental ones. This was the 'blood and custard' period of carriage colours. There were only eight actual locomotives in use between 1947 and 1957, 10000 & 10001 LMS Co Co diesel electric 1600hp, 10201-10203 SR/BR 1Co Co1 diesel electric 1750/2000hp, 10100 1D1 BR/Fell diesel mechanical, 10800 BR/Brush Bo Bo diesel electric 800hp, and "Deltic" English Electric Co Co diesel electric 3300 hp. All except the blue "Deltic" were in boring black colours.
The Modernisation plan started in earnest in 1957/1958 with a Pilot Scheme consisting of a number of small classes to try and find out the best ones. Some were more successful than others.
In this early 'Pilot Scheme' phase and into the days of steam most diesels were turned out in green livery. At the same time most carriages were in maroon livery. The classes that were seen as successful eventually became the following under the TOPS system and got the Corporate blue livery.
20, 25, 26, 27, 31, 33, 35*, 37*, 40, 42/43 (Warship), 45, 46, 47* 52* and 55* (* technically the 35, 37, 47, 52 and 55 post dated the Pilot Scheme but were well established during the transition between steam and diesel)
In 1968 steam was abolished and diesels (and electrics) were in command. Blue for locomotives and blue and grey for carriages became the dominent colour scheme. This then remained the status quo until the 1980s when new Intercity, Network South East and Provincial stripy liveries started to appear. Many of the older 'Pilot Scheme' diesels gradually disappeared in this time.