It looks like the local authorities, and TFL, actually grant a parking exemption to security vehicles collecting or delivering cash: https://www.stratford.gov.uk/files/seealsodocs/12206/Microsoft%20Word%20-%20wcc%20sdc%20cancellation%20policy%20draft2.pdf, see page 17. so as long as they are not parking indiscriminately or causing an obstruction, they are not parking illegally.
@bq started
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It looks like the local authorities, and TFL, actually grant a parking exemption to security vehicles collecting or delivering cash:
https://www.stratford.gov.uk/files/seealsodocs/12206/Microsoft%20Word%20-%20wcc%20sdc%20cancellation%20policy%20draft2.pdf, see page 17.
so as long as they are not parking indiscriminately or causing an obstruction, they are not parking illegally.