On Thameslink, the Peterborough/Cambridge services that don't go through the core start/terminate at King's Cross instead of using the usual platforms at St Pancras "Low Level" (as can be seen by
searching for Thameslink departures at Finsbury Park to/from Stevenage) so no consistency there, but perhaps there are operational reasons precluding terminating inside the Thameslink core (especially with other services passing through throughout the night) that don't apply to Crossrail. (On the other Thameslink branches it looks like trains that don't run end to end run always run through the core before terminating/after starting, instead of terminating/starting short of the core.)