The best dating sites for finding a serious relationship in the UK

Anyone who's been doing the online dating thing for a while knows that there's hookup culture and then there's long-term relationship dating culture. Most dating sites have a mix of both, and after living with online dating as an increasingly ubiquitous option for the past 20 years, the general public mostly sees dating sites as a normal means to find casual dates or a hookup.
Where does this leave you if you're looking for a serious relationship or something long-term? What if you just don't wa