There is no specific policy regarding having a TV or cable/satellite service when renting an apartment in London. It is generally up to the landlord or letting agent to specify any requirements or restrictions regarding the use of TVs or cable/satellite services in the rental agreement.
However, it is worth noting that if you plan to watch or record live TV broadcasts in the UK, you will need to have a TV license. This applies regardless of whether you watch TV on a traditional television set or on a computer, tablet, or mobile device. You can find more information about TV licenses on the TV Licensing website.
If you are interested in subscribing to a cable or satellite TV service, you may need to check with the building management or the landlord to ensure that it is allowed in the building. Some buildings may have restrictions on the installation of satellite dishes or cables that could affect your ability to get cable or satellite TV service.