I priced your dates at €5724 in J (I can't see the LHR-DUB on Aer Lingus). I did exactly this trip in the summer ex-DUB, it was best for me to fly into TPA and out of MCO. Unfortunately you don't have the same level of ex-EU options going to TPA or MCO as both use LGW (unless you want to change airports in London).

Agree with @chris1922Mk2 about Avis Preferred - collection at both MCO and TPA is a nightmare without it.

Hiring a Chevy suburban would probably cost £1,000 for 15 nights so deduct this from your price and you are talking ex-DUB returns to Florida for about £1,200 each which is pretty decent but not spectacular.