Route clearance is not down to SWR but actually Network Rail.
2014 is 10 years ago, just because there were cleared then doesn’t mean the gauge of the network hasn’t changed, the whole Windsor side has been resignalled since with new trackside equipment.
Network Rail no longer so so clearance by proxy, ie it’s a similar vehicle to another one, all stock is now classified in its own right.
Because of the change of use damper Network Rail now class this as a new unit, nothing to do with delaying tactics by SWR.
You might not like the way the modern railway functions but there’s no way the TOCs can circumnavigate this, they are at the behest of the system operator.