Surely the return time is likely to depend upon where the holiday started from, isn't it? People travelling back to Newcastle will probably get home a lot later than those travelling from Barnsley (which are both places that National Holidays operate from):
https://www.nationalholidays.com/itineraries/3193-london-and-harry-potter-studio-tour?currentpage=1&areafilter=
Unless you're lucky enough to find someone on here who has been on that weekend break AND who has travelled from the same point that you're considering booking from, I can't see how you'll be able to get an accurate answer here. This would seem to be a question where a phone call would be the best way to get an answer: 0844 477 9990 (0800-2200).
However I can tell you that, when I was working in Folkestone, I stayed in a massive hotel which was packed full of hundreds of people who were taking a break with National Holidays. Because I was carrying out interviews on a path close to the hotel, I got to talk to many of those people and every single one of them had nothing but praise for National Holidays. (So much so that, as soon as I got home, I I investigated whether they operate from around here, as I was keen to book with them. Alas, they don't!).