I can't answer you inquiry on QM2, but like you I recognise the Butlins future of cruising. I was fortunate to be invited to the pre launch of Holland America Eurodam the other week & if you've not read the review & seen the slideshow link put on by one of the other contributers to this site, give it a look. This ship reminded me of how cruising used to be & may appeal to people like yourself who are looking to get back to the days of quality not quantity. Regards.
As you have only had a comment and not an answer try the search engine on the Reviews page find Cunard and then the ship you want you will probable get the info you want.There are other 5/6 star lines which may interest you,best of luck?
The QM2 spa is lovely, I cannot compare it to other ships but my fellow passengers rated it better than P&O and Celebrity. The main spa area has 2 saunas, steam room, therapeutic showers, foot spas, hot tub & small pool with swim against the current and a couple of other effects. Lots of beds, robes, slippers, drinks etc. Access to the spa area is payable but a more than one day pass is good value. Many treatments are available.
In your cabin a bathrobe will be provided you do not need to take one.
Will look out for your view on how you compare P&O to Cunard.
Enjoy your trip. Karen