Are you saying it would be too hard to test while in London? If so, you can just test and the port still, but pay for it.
I know many disagree with the change because of the increased risk, but I personally would have really appreciated testing before going to the port while in Rome. It would have made the whole embarkation process part of the fun and excitement, instead of a long stressful process. Even though we all self-tested negative before going, there was still the thought of "what if" the official test comes back positive in the back of our minds and we couldn't really enjoy the excitement of arriving at the port. I know the counter to my opinion is, "how exciting would it have been to catch COVID on board?" But I am still not sold that the antigen testing at port significantly reduces that risk for various reasons.
This will also give some the option of testing before they even fly to Europe - probably cutting it a bit close for my taste, but certainly possible.