Do you mind if I ask how long you've been in the UK? If it's been a little while, it might be the difficulty of switching from "new and exciting" to "crap, I'm stuck here for good." When I've lived as an expat before, I've noticed that around 6 months, 2 years, and 6-8 years into a new country, I hit some massive down times. That plus a terminally ill MIL is a lot.
Do you have anything that you do on your own and isn't connected to your spouse? I think that's SO important when living in a new country. Even if it's just volunteering with the Silver Line (making weekly phone calls to an isolated senior citizen) or helping with a garden or something, it's nice to have something that's all your own.