With regards to transfer home for withdrawal of ventilation - it is possible to move sick kids home or to a local hospice if it's the parents wishes, it's something that I have been involved in and a lot of parents and families find comfort in being at home with their little one.
However, it does come with logistical challenges and all involved need to be in agreement with the plan of care, with acceptance from the parents that their child will die (usually quickly) after life support is removed. I suspect this is one of the biggest factors in this case - that parents still need to accept and agree with a plan for withdrawal and work with the MDT for Charlies benefit. Threats made against staff and the suggestion that HCPs won't be allowed into the house certainly do them no favours.
Transporting a sick child takes time and skill - most NICU and PICUs in the UK work with a neonatal or paediatric transport service. In London, this is CATS - a team that works independently of the local unit and aren't based at GOSH. The staff that make up the CATS team either work full-time for CATS or are PICU staff who rotate onto transport. The CATS team operate an acute (emergency) retrieval service and a non-urgent (repatriation) service - they are an incredibly busy team. They would need full compliance from the parents with regards to moving and a time-frame as they will have other jobs (as brutal as that sounds) to complete that day. Or they have to put on a specific team just for Charlie.
The equipment needed can be cumbersome - iirc, they live in a flat. If the flat does not have lift access, the logistics get more difficult. You cannot wheel the equipment straight into the house - you would have to hand ventilate Charlie and carry the extra bits needed.
Then there is the media frenzy and SM outpouring that could compromise the transfer - I wouldn't particularly want to transfer a baby surrounded by an audience shouting "justice for Charlie" and waving their blue balloons. His journey home would be closely watched, potentially requiring Police involvement - another logistic and extra resources needed.
I do wonder if the involvement of yet another party in withdrawal of care for Charlie adds to the legal complications regarding this case. There is strict paperwork and procedures to follow for the transfer for palliative care and plans for DNAR need to be watertight if he was to be moved elsewhere for withdrawal of care.
Most importantly is Charlie in all of this - what is best for him? All of the above can (and have been) overcome to allow him to return home, but until the parents fully accept and agree, it cannot progress forward.