As GG indicates, all energy suppliers are struggling to keep within the price limits set by Ofgem at the moment. (That's why about 40 suppliers have gone bust and even some really big firms, such as Shell Energy, now look set to leave the market). So they're all charging the absolute permitted maximum at the moment.
Even if there were variations in what energy suppliers were charging, what customers wouldn't actually pay wouldn't change. For example, the current Ofgem price cap is 67p per kWh and that's what all suppliers are now charging. However the UK government has set its own price cap of 34p per kWh, meaning that for every kWh of energy that's used, the customer pays 34p and the government pays the remaining 33p. If, say, an energy supplier was able to cut their price to 57p per kWh, it would be the government that saved 10p on every kWh, with the customer still paying 34p.