Second the Woodland Spa recommendation. It blows all the other spas in the area out of the water (pun intended). But it might depend what you want from a spa.
I want lots of warm bubbly water to lounge around in, a nice lunch and somewhere for a snooze after lunch. I always go alone, a couple of times a year and I never have treatments. The Woodland ticks all these boxes for me.
The Titanic didn't have enough bubbly water for me and the restaurant, which was open to people not using the spa, was right in the middle, so you had what was for me the slightly weird situation where you had people in dressing gowns wandering around amongst restautant guests.
I also like Rudding Park, the rooftop spa is spectacular, but it's probably more expensive and it's a bit full of 'look at me' Instagram types, or it was on the day I went.
In the north of England, my favourites in order are:
Woodland,
Rudding Park
Center Parcs Sherwood - pretty good but loses points because you're not supposed to use your kindle and the outdoor hot tubs are usually full of people who Never Stop Talking
Titanic