Network coverage in Cuba can be sketchy, it’s better in the large cities, and resorts, in rural areas there is very little if any coverage. You should check with your mobile provider before leaving the UK, but be warned, making and excepting calls, and using mobile data in Cuba can be eye wateringly expensive, most hotels now have WIFI access in Cuba, you will be better off connecting to this and making and excepting calls via whats app.
Cuba only has on mobile network operator, Cubacel. Tourists can purchase sim cards that are pre-loaded with data at Havana airport (subject to availability) or before you go from Suena Cuba, sim cards can then be collected at Havana, Varadero or Holguin airport, more information can be found here: https://www.suenacuba.com/tourist_sim_card_cuba_buy your phone should be unlocked (SIM-lock free) to enable you to use this.
This is a guide only, all information is subject to change, Love Cuba does not work with Suena Cuba and accepts no liability for any transactions.