All of the train operating companies (particularly those with routes into and out of London) want to offer greater capacity on their trains but they're having to wait until Network Rail can increase the platform lengths at many stations, so that they can run longer trains. (Lengthening a platform can often be a major task, as it can involve relocating sets of points, signals, overhead line infrastructure or even moving buildings). They're also reliant upon the train leasing companies actually having the required rolling stock available. (Orders for new trains often need to be placed several years in advance). They're also looking forward to the introduction of new signalling systems, which will allow trains to run closer together.
The number of journeys made on Britain's railways has increased by 83% over the past decade and is expected to double again over the next couple of decades. Rail travel in the UK is a victim of its own success; it's increasingly hard for Network Rail, and the train opearating companies, to keep up with the demand for their services.