Hi and welcome to V-Flyer.
First of all, relax and look forward to your trip. We will do your best to let you know what to expect so that you will have as smooth a flight as possible.
First of all, I have no experience of flying on Virgin Atlantic out of Manchester but others will be along to help you there.
With regard to the 747 it is my favourite plane. However I should say that the ones that fly out of Manchester and Gatwick (they are the same planes and interchange airports) are currently in the process of being upgraded and the first one is due back any day now. The second one will follow immediately but it is unlikely that it will be back before your trip. You may or may not be on that plane (the chances are that you won't) but don't let that stand in the way of your enjoyment. The 747s on this route are some of the youngest in the fleet but are hard workers and some are beginning to look more than a little tired. Please don't let that put you off because our experience is that a good crew more than makes up for it. Because of the re-fits the seat map is not available so you won't be able to select seats until on-line check-in opens 24 hours before the flight (although if you look on the website a couple of hours before then you might find the map open) If you can, select a seat on the upper deck but you may well find the seat has been pre-selected for you. That seems to be the report back from recent flyers to MCO.
There is a new meal service for PE starting on June 1st so keep your eyes on this website as people will start to report on it pretty quickly.
At immigration they are not interested in whether you are on a package holiday or not. Presumably you have filled in your Advance Passenger Information and have your ESTA? They will simply ask you the purpose of your visit, probably how long you are staying and then photograph you and take your fingerprints.
Finally, just to reassure you, I too am an older single lady (well widowed actually) and have never had a problem with my flights with Virgin. If you feel a tad apprehensive just mention it to your cabin crew and they will help you.
Finally, finally If you click on to the Toolbox tab at the top of the page and then go into Trip Report Index you can read some trip reports on your flight to get some idea as to what to expect. Also look at the VS75 but bear in mind that the newer ones are not on a 747.
Enjoy your trip. You'll be fine