I agree NGM, things are probably worse than I indicated, but I was not attempting to define the degree of problems previously, only to comment on some aspects of the problems.
As you say, it is outrageous and anti-competitive of BT to prohibit you being able to divert using a cheaper call carrier. However, that is typical of what we have come to expect from BT, solely because the supposed regulator never does what they should do to stop these anti-competitive tricks.
What you could do of course is to purchase a diverter stand-alone product, which would then not incurr any charge for a star service, and could be used to divert via any carrier you choose. The only problem I believe you will find is that there are now no diverters approved by BEAB or whoever the current approver is for connection to the network. (This was yet another trick to destroy competition, to withdraw the possibility of official approval for these.) It may be difficult to buy one any longer in the UK since it may be actually illegal to sell them in the UK now. You might have to source it on the Internet.
If you do get one you would not be legally allowed to connect it to the PSTN, although of course in the circumstances people do so illegally, which is hardly surprising now that the possibility of approval has been taken away! Your only cost then is essentially the up-front cost of the diverter and its maintenance. Of course if you don't already have a second line that would be an additional cost which could tip the balance against your own diversion, but you could consider using a PAYG mobile permanently powered-up from the mains which would not incurr any additional line cost.
At a quick look there seem to be some other telcos offering diversion services, but it is not altogether clear which of them offers this for GNs, but one or two may be cheaper than BT?
A quick trawl came up with the following cos who offer diverter units:
http://www.allproducts.com/manufacture1/gida/supplier.htmlhttp://www.woodtel.com/http://www.design2000.com.au/home/home.htmlOn one site there is even a simple circuit to build your own!
http://www.hackcanada.com/blackcrawl/telecom/p50-09.html