Which suppliers offer flexible energy purchasing?

The majority of commercial and industrial energy suppliers offer some sort of flexible energy purchasing. However, it is the larger players in the UK energy market (British Gas, Corona Energy, Dong Energy, E.ON, EDF Energy, Engie, Gazprom, Haven Power, Opus Energy, RWE Npower, Smartest Energy, SSE, Total GP and Wingas) who offer the most competitive and attractive products.