The relevant point is where the passengers were security screened; was it in the UK or elsewhere ? Passengers arriving at a UK airport directly from another UK airport will be allowed to mix with security-cleared departing passengers whilst passengers arriving from outwith the UK will be kept completely separated until landside, i.e. have passed through baggage claim and customs.
The same basic rule normally also applies to transit passengers at UK airports: those in transit from other UK airports need not be screened again as long as they have not been able to mix with other non-screened passengers, or with ex-foreign passengers. All transit passengers who have arrived from abroad must be UK screened before being allowed to board their onward flight.