The short answer is that no crew should ever attempt to undertake a rescue [particularly at night] without a great deal of practice, onto an unsurveyed site in an environment where there are unknown thermals.
The need for a roof rescue appears to be once in a lifetime across the UK. So you might consider training up one crew [police, fire or coastguard perhaps] to undertake the mission that they might have to undertake just once in their career. If that crew is untrained for the mission they should not promote the capability because it will just add three more to the pyre.
Simply to pre-survey the rooftops in London during daylight will take weeks and that needs to be done every year without fail to guard against some new aerial, some new wire...... now multiply that by every city in the UK with high rises..... 6,500 is a number that has been bandied about.... and the planning alone is breathtaking.
Then you need to consider how you lift 50 people off a roof using a 20 seat helicopter.
The plan in New York [where they have some experience of planning and training for just such an instance] includes interting armed officers first to clear the rooftop of debris then to control boarding....
During 9-11 the trained NYPD crew in a Bell 412 considered a roof landing in daylight. After one tower was hit both towers were covered in smoke and only a tiny part of one tower could be considered a landing spot clear of smoke shrouded wires. We now know there was no way anyone could get through the locked roof doors of the WTC but clearly there were other issues that suggest that roof rescue is fraught with so many questionmarks.
Perhaps, and it is a big ask, the only people who might have the experience [?] would be the remaining military SAR as they might be training to undertake risky rescues that might be not too different from high rise .... but that does not remove the need to survey 6,500 roofs and make it readily available 24/7.
This is not my personal opinion, it is simply a condensed version of an experts presentation - a man who was at the WTC on the day.