Also, the number of experimental classes at the start of the diesel era was silly, as many non-standard types were only in service for a few years before being withdrawn and scrapped.
Many of the experimental classes of locomotive were ordered under the Modernisation Program. Ideally, BR would have followed the German practice of electrifying all main lines while keeping some steam locomotives in service for the time being to avoid over-investment in diesels. However, by the time the Modernisation Plan was published, BR was running at a loss and decided that costly and wasteful 'panic buying' of lots of different locomotives was the way forward. However, this was very inefficient and wasteful because many steam locomotives, like the Standard class 5s, were only withdrawn after about 10 years of service; in fact the Standards were originally built to last into the 1970s.
BR also faced political pressures in that British locomotive builders were struggling due to lack of export orders. While it would have been cheaper to order, and then license produce (in fact, the Swindon-built class 42s and the NBL-built class 43s were scaled-down versions of the DB V200 class) American diesel-electrics or German diesel-hydraulics, that was considered politically unacceptable. Therefore, BR had to order lots of untried designs from a variety of manufacturers, some of which were complete disasters (e.g. the class 14, class 17, class 21 'Baby Deltics', many of the first-generation DMUs) that lasted for only a few years. Having lots of different classes meant that due to different coupling systems many couldn't work in multiple.
The Modernisation Plan failed to take into account changing requirements for the railways. Quite sensible options of reducing the costs of operating branch lines like line-singling, building 'bus stop' type halts, abolishing freight traffic and de-staffing rural stations weren't considered, even though BR was, IMO, heavily overmanned. Instead, the class 14s, 20s, and other type-1 diesels were intended as a direct replacement for steam locomotives on local passenger trains and pick-up goods, even though pick-up goods trains by then were becoming extinct because road haulage was cheaper for small consignments.
Finally, the Regions had far too much autonomy, IMO, of what they ordered. For instance, the WR went for diesel-hydraulics, the LMR went for big diesel-electrics, while the SR bought 'Thumpers' and the 73s. This duplication was pretty expensive, given that under the National Traction Plan of 1968 the diesel-hydraulics were to be gradually withdrawn even though many had only seen 10-15 years service. If BR had been more sensible with rolling stock procurement (for instance, by only ordering a few types of diesel and electric locomotive for various duties, and maybe only two or three classes of DMU for rural services), retained some classes of steam locomotive until the 1970s in order to avoid over-investment in diesels, and considered more carefully the role of the railways in years to come, then maybe BR wouldn't have become so indebted the government in the early 1960s came to view BR as a liability, which made the Beeching Plan, at the time, an attractive prospect.
But maybe hindsight shouldn't be taken so seriously...