I disagree, most things you do as a couple, you do as a single as well, trips, vecations, going to the movies, eating out at resturants etc etc ... how many nights do you spend in if you are single? I didn't sit at home much when I was living by myself, I was out all the time doing things, and that often involves spending money. Being in a relationship I find myself perfectly happy spending nights in with my girlfriend.
Living together is cheaper then living by yourself thats just how it is, when I first moved out I was living with a friend because it IS that much cheaper, you pay half for rent etc, I do not understand how you can spend more money in a situation like that. I just do not comprehend your math here, unless you just stay in all the time doing nothing. Almost everything costs more when you need to pay for it by yourself, it adds up.
I'll give you an example, our rent is 5000 SEK, by sharing we each "save" 2500 SEK each month just on rent! Thats 225£ each month that we can spend on doing things solo or together or buying things, or putting money away for traveling.
It's a commonly known fact that single people in general got less money to spend after bills because they got more expenses. Thats how it is in Sweden at least, I do not know why it would be different in the UK.