The TFL train, departs from the same platform as the Heathrow Express, is much cheaper and goes the same route. It just stops along the way. So it takes 30 minutes instead of 15. Here i is the info:
https://www.londontoolkit.com/travel/heathrow_connect_train.html
Also wanted to mention that I took the EBTA in September a few years back and it was cold and windy. The weather wasn't bad, but it's the middle of the Atlantic so overcast. No one was in the pools or up on decks. There were blankets at the outdoor seating around the Cove Cafe. It's not like sailing around the Caribbean. I did not have a veranda but if I did the only times I would have sat on it were when we docked in a couple Canadian ports because then the wind had stopped. I know there was another thread about this but I can't find it now. I honestly can't imagine sitting on the veranda on the sea days - so bring a blanket or jacket if you plan on this!