I agree with BigJohn.
Use Call1899 for daytime calls to geographical numbers 01 & 02.
Switch your call provider from BT to Planet Talk (link in previous post) and use this for evening & weekend free minutes with no monthly fee. This will switch just your calls to planet talk automatically without the need to dial any override numbers.
You would still need to use override code 1899 when routing your daytime calls via Call1899.
You could get an Orchid Dialler to route your calls during the daytime automatically to Call1899, and route the calls during the evening & weekend via Planet Talk (ie CPS provider). An Orchid dialler can be setup to route different type of calls depending on time of day, to practically any provider of your choice.
See
this thread on MSE forum for 20% off an orchid dialler (if needed).