Firstly, that's just not true and it's especially unlikely to be true for a machine whose sole purpose is to convert fossil fuel into usable energy, and in this case we are only really using the shell of that machine.
Well I think we can agree that it's cheaper for GBRf. But is it better value for the people who breathe the air pollution? And isn't it a bit of a fiction that it's cleaning up existing locos? I think we all know that if the 69s are successful they will see lots more use than the donor 56s they replace would have seen.
You might say, well, but increasing capacity will help take traffic off the roads and I can agree with that in principle but rail emissions are not improving anywhere as fast as lorry ones are. There needs to be a long term plan to improving emissions and allowing these kinds of loopholes goes no way to achieving that. In fact it removes the demand that might bring a properly compliant new design to market.
In short your answer is here:
No one is going to bring a new diesel only design to the UK given the 2040 "diesel only" ban.
GBRf need additional locomotives but not enough to justify building a new fleet with a compliant engine that, by the time it gets into service, would have a lifespan of 17/18 years. So rebuilding older locomotives with more compliant engines makes sense for them.
The long term plan will likely come towards the early 2030s when the freight operators start looking at what can will replace the Class 60/66/67/68/69/70 fleets. Hopefully we will have a plan for more electrification in place, so more locos like the Class 88 (bi-mode) or Class 93 (tri-mode) will be looked at; but we will still have a large diesel fleet that needs replaced and options will have to be looked at. There are better incentives for manufacturers to develop and pitch something for the UK market when they have potentially large orders to win.