Take plenty of sterling notes. Went to Havana and Varadero last month for first time. Long queues at the few currency exchanges in Havana. You cannot withdraw money on your debit card and ATMs very few and far between anyway. Most places won't take credit cards and our hotels would not exchange cash on a debit card, only for cash. There is a currency exchange booth in the marina at Varadero which will let you draw currency on a U.K. Debit card.
Cuba is still basically a cash only society, and this is not well flagged up in the brochures.
Glad we had plenty of cash on us.