Getting to Cuba is easiest with the direct Virgin Atlantic flights from Gatwick. They have 2 flights a week to Havana & 1 a week to Varadero, which gives the ideal option to fly into 1 & out of the other.
There are other airlines that operate via several European cities as well as via Colombia with Avianca. There are now flights coming into Cuba from various cities in the US, so going via a US city may become an option one day- at the moment there are only 12 acceptable travel reasons to fly to Cuba from or through the US, & none of them are a simple “going on holiday” reason.
Whilst in Cuba the best way to travel between areas is by private transfer, the perfect option is to have a sightseeing transfer so you get to make stops & visit interesting places en route. Fir example from Trinidad to Varadero the sightseeing transfer will visit Sugar Mill Valley, Che Guevara Museum, Mausoleum and Tren Blindado in Santa Clara. Self-driving is also an option & there are also domestic flights that can be taken.