but which shieldbug will it become?
No interactions present.
I'm pretty sure this is a final instar nymph of the Forest Shieldbug, Pentatoma rufipes, which is illustrated well at:
The shape and colour of the pronotum, plus the yellow spots around the edge of the abdomen, seem to be the most characteristic features.
Entomologist and biological recorder
I would second the above, a final instar nymph of P. rufipes
National Recorder for Terrestrial Heteroptera (shieldbugs & allies)
Lat/Lng: 50.9, -1.7
OS grid ref: SU2413