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. 

I have a Big Agnes lost ranger UL which is a 2 part system, one of which is like you describe, the other a regular bag, or you combine both for maximum insulation. 

I think going all in like this is a bit much personally, but I do have a zenbivy sheet and quilt, and I found that for the specs etc. the value wasn't awful when I purchased, at least for backpacking equipment factoring in weight etc. For moto camping alone, it's a bit overkill. Breaking it down a bit: I think the pillow looks a bit overpriced, but if it's super good maybe not awful. The mattress also looks a bit overpriced to me - compared to a similar Nemo or Big Agnes. The dry sack is overpriced IMO, and not really necessary unless you don't have waterproof luggage. I just pack my quilt in wherever it'll fit to take up all the extra space. The quilt itself I think is pretty solid, 10° is pretty cold but maybe makes sense depending on where you're located, and the integration with the sheet is super nice IMO. The Big Agnes lone/lost ranger are competitive options that are priced generally similarly for context.