*see UPDATE below* British Airways together with their Oneworld partners (Iberia or American Airlines) offer good solution from the UK airports to Miami next spring and beginning of summer 2018. Return flights from Glasgow, Manchester or Edinburg are now available from £302 incl. all taxes, in-flight meals and checked bag. If you are flexible and want to fly on board of British national airlines for an affordable fare this is good opportunity..
*UPDATE*.: Adding one more interesting option from Ireland (Dublin, Shannon, Cork) to Miami. Return flights are available from €333 either on British Airways or Virgin Atlantic.
You will find the cheapest British Airways flights from the UK to Miami in May and June or September and October 2018. Max stay 12 months. BA special promo sale is available till 22nd of November. YOu can secure your hotel in Florida using service of Hotels.com. Also you can consider current promotional offer of Booking.com granting $40 cashback after your stay.
Booking sample of latest promotion of British Airways: Cheap return flights from the UK to Miami from £302! In this case you would depart from Glasgow to Florida on 10th of June. Your return flight from America to the UK would be on Sunday, 17th of June 2018.