Ah, 1andrea - once you go business class, you can never turn right again ...
I fly BA business quite regularly for leisure and it's a decent product. If you're flying from LHR then you'll get a better seat and IFE system than on the flights out of LGW. On boarding you'll be offered a choice of drinks (OJ, champagne or water) and not that I want to influence you, but it's always champagne o'clock somewhere in the world. I'd have a fiddle with the controls when you board - they're pretty intuitive and the pictures tell you how to make it go flat, recline etc. There's a kitchen for snacks inbetween meals - be warned that it does tend to get raided but I've always found biscuits and chocolate to eat there.
Flying into Miami means that you'll be able to pre-select your food a month in advance. I haven't used this but I'm told it's pretty easy. My only piece of advice is not to go for the beef unless you like it very well done.
The BA flights to MIA leave from Terminal 3 so you will probably need to transfer from Terminal 2 but that should be straightforward. At LHR, you'll be entitled to use the Elemis spa. They've got a good range of treatments but it does get booked up very quickly. How long have you got as a connection time at LHR? If you're getting in early and have 3 or 4 hours plus, you should get be able to get an appointment but it all depends on if you're flying at peak holiday time or not. If you do get to use the spa, I'd recommend a facial as you usually get free product samples afterwards.
I think BA are currently sharing their lounge at T3 with Emirates so it can get busy but when I was there in June there was no problem in getting somewhere to sit. The lounge does buffet food - breakfast is usually bacon rolls, porridge, pastries and vegetarian breakfast rolls (usually mushroom if memory serves). The lunch and dinner options can vary and have a bit of a school dinner feel - chilli con carne, soup, pasta - that kind of thing. Some people absolutely hate the food but I've never had a problem with finding something decent to eat (although don't overdo it because you'll be eating on the plane too).
The bad news is that so far as I know, Aer Lingus don't give you access to their lounge at Dublin unless you're a BA gold or silver card holder. It's worth asking in case that's changed now that BA is buying them out, but it was definitely the case last March.
Not sure from your post if you're flying from Miami to Orlando. If so, are you doing it on American Airlines because if you are, then if you aren't already a member of the BA Executive Club, it's worth getting a membership card because you'll get enough airmiles from the trip (including the Aer Lingus flights) to get a return to Europe. You might also get enough tier points to hit bronze.
Hope that's all useful and that you have a fabulous trip!
1F