Hello
westham
The ILS critical area is an area of defined dimensions about the localizer and glide path antennas where vehicles, including aircraft, are excluded during all ILS operations. The critical area is protected because the presence of vehicles and/or aircraft inside its boundaries will cause unacceptable disturbance to the ILS signal-in-space
my emphasis
As per the definition, if aircraft are in this area, it cannot be a critical area
The definition doesn't say that at all really. It states that vehicles and aircraft can't be within the critical area if there is an aircraft making an ILS approach. Just because there is a vehicle/aircraft in the critical area it doesn't cease to exist. Notice how the definition states the critical area is protected and not prohibited.
Therefore as Gonzo said aircraft can transit / sit in the critical area as long as there isn't an aircraft using the associated aid, the GP in LHRs case.
Similarly you could have an aircraft / vehicle sat in the GP critical area as long as the approaching aircraft wasn't making a GP assisted approach, i.e. LOC/DME, RNAV, MLS, Visual, etc.