According to the AIP (chart AD 2-EGLL-8-10) the LLZ/DME procedure starts from 2500'. Since this is the case, you guys in the Tower would only have to guard the glidepath from 7.5dme if we used 2500' as the platform altitude for the 27L ILS when TEAMing. This won't cause a problem with deemed separations as clearly 2500' is OK for the LLZ/DME so it must be OK for the ILS.
As to adding workload to FIN. I wouldn't like to comment on whether it was better to increase the workload of someone who has spent the night in his own bed or taken a long sleep in the bunk room/vacant office, or whether it would be better to give more work to 2 guys who are arriving after spending a night out of bed, coming from a timezone 6 hours away, perhaps snatching an hour of sleep in the seat, then staring at the rising sun for 3 hours. (More seriously, the GS could act as coordinator to keep FIN's workload manageable).
G W-H