Saving with existing BT contract

I live in an area which went over to fibre just over two years ago and had to go with BT because they were the only provider offering ultra high speed via fibre to home (fibre to cabinet was not available in our village location). All went fairly well but I became concerned because we were paying £71 a month for 300Mbps incl line rental. I was just accepting this as no one else offered the ultra fast speed. Today I happened to notice that BT were advertising ultra fast fibre (ave 300 Mbps) for just 49.99 a month for two years. I got online with one of the reps at their contact centre and he confirmed that I could take out a new two year term at that rate (it is 52.99 after first two years) It seems that very unfairly BT do not pass on price reductions where they occur unless you take action at the end of your initial contract. It looks as if I shall be saving a whopping £20 a month without changing company or reducing the level of service. I will certainly look again when the new contract is due to end.
I found this particularly frustrating because knowing that no other provider could offer me a fast fibre connection (I had tried several) I had not considered comparing prices!