There is some information
HERE and it sounds as if some of the acceptance may be through reciprocal agreements with other network(s):
In the U.K., UnionPay cards are accepted at 40% merchants, covering various consumption scenarios such as catering, travel, shopping and entertainment. In particular, Bicester Village, Harrods, Watches of Switzerland, Liberty, Fortnum and Mason and World Duty Free at heathrow. As well as a number of restaurants and hotels beloved by the local people, including: Bright Courtyard restaurant, Hakkasan, Dishoom, Wright Brothers, the Ritz London, Mandarin Oriental Hyde Park, Four Seasons Hotel, Landmark Hotel.
When using co-branded cards, you are advised to choose UnionPay as the routing option to be benefited from the special offers and services provided by UnionPay.
Note:Some UnionPay card acceptors or acquirers route co-branded cards to corresponding card shcem according to the card BIN, which means co-branded cards transactions aren't routed to UnionPay without selection by cardholders and merchant cashiers. Please refer to the transaction results for the information about transaction routing.