Lost Ranger 3N1 15°

What are you planning to use this for? Is your partner a back or side sleeper? These bags have very different weights and temp ratings. The Echo Park bag is v heavy (nearly three times the physical weight of the Greystone one), so would be awful for a backpacking camping trip but might be lovely for a car camping trip (albeit, I really doubt you need a bag rated to -18C for UK camping!). I use a Big Agnes Bag (Lost Ranger 3N1 15), which seems to be a lightweight version of the Echo Park bag, and I love it. I like the padlock system, which anchors it to the pad, and the side-sleeping bag/quilt, but those might not be important to your partner. I also primarily backpack, so weight is crucial and it’s excellent to be able to ditch the bag in the summer, halving the overall weight of the system.

I have a Big Agnes Lone Ranger 3 in 1 sleep system and have used it in British Columbia over summer/fall hiking and took it to Mt. Kili this past December and was warm every night up there. Loving it so far, and both the bag and quilt compress down well for packing. Whatever you go with, best of luck on the trek to Kili!! It was amazing

We have a Big Agnes something or other 15 degree down bag we bought for around $300...10 years ago. It's great, although more like a 30 degree bag. Size / weight is about equal to two sleeping bags. We also tried out Feathered Friends' Spoonbill which is well out of your price range but might be interesting to look at to see how an actually warm two person bag looks.

 So the problem with any normal two-person sleeping bag is that there's an air gap between you if you don't sleep snuggled up.  The Big Agnes we have has a small flap that covers that up. We have also stuffed clothes in between us when it was colder. That works for 15 to 20°, but not colder. The reason I said the spoonbill was interesting is that it has individual draft hoods for each person so you share warmth but actually can't cuddle. 

Kelly cosmic down bags are amazing bang for buck. REI has generous return policies as well, so if you get into a bag you don’t like, you have the option to return it for a year. More expensive bags/huge discounts don’t necessarily mean they are going to fit your needs. I currently sleep in the Big Agnes Lost Ranger, which is a 3in1 sleep setup. It is a quilt and a sleeping bag, which means you can sleep with just the quilt, just the bag, or if it’s cold enough you can nest the sleeping bag inside the quilt. It’s on the more expensive side, but it could be beneficial looking into a similar setup. I would however recommend investing in a nice sleeping pad that has a decent R value. I currently sleep on the Big Agnes Rapide SL Insulated (regular) and I LOVE IT. it comes in all different types of widths and lengths. I’ve slept on it for 2+ weeks in the Yosemite side country and I’ve never had a single issue with it. long story short, i’m a huge fan of the Kelty Cosmic Down line. there are also a few REI branded bags that may be worth looking at.

I'm in central Idaho. I use the Big Agnes Ranger 3-1 15 Degree bag for my elk season that runs late October into November. The double bag combo works well for keeping warm into the 10-20 degree overnight temps. I pair it with an inflatable pad with an R value of 4. A good warm pad makes the biggest difference for me. I'm not sure how you are, but if I don't get good sleep, my motivation tanks, especially as I get into the later days of my hunt. If you're not taking a hot tent you should get a bag that will keep you comfortable in freezing conditions.