To avoid roaming charges from your own Carrier, if arriving by Air, you should find SIM vending-machines in the Arrivals Hall (often within baggage reclaim - prior to Customs) where you can purchase a prepaid SIM for mainstream Carriers for either a mobile phone - or a DataSIM for tablet computers. EE, O2, Three and Vodafone are among the most popular. While waiting for your checked baggage, this is an opportunity to choose a suitable plan for your visit.
Recent model iPads, supporting eSIM, can also support purchase of short-term data plans directly from the iPad. Within the UK, choices will include GigSky, ReadteadGO and Truphone. These tend to be more expensive, but can be activated from almost anywhere using a Credit Card.