I looked for some guests of ours this morning and there were indirect flights from Heathrow for around £620.

BA from Gatwick to Tampa – Direct out on 31/3 back 15/4 £785
Orlando prices were higher and Virgin flights not yet out.

Have recently found Virgin to JFK and then JetBlue to MCO works well on price.

As it is Easter it will be a lot more expensive. good luck