As noted some way back in the thread, I've both the Borah Gear cuben fibre and the Borah Gear ultralight bivvies (in fact Cath has an ultralight bivy as well). There's 45g difference between the two, the main difference is the material for the floor: cuben (obviously) and silnylon respectfully. Both models come with a section of noseeum (what the Americans call midges) or midge netting for the neck area upwards but all ours are slightly custom in that the panel of midge netting is more of a letterbox with the actual head end being Argon to provide a bit of protection from spray. The bivvies have a grosjean loop that lets you attach a lifter to keep the netting away from your face.
The material for the upper, a couple of weights of Argon, isn't waterproof so you can't use the bivy on its own. Also with having the netting panel it was never going to be waterproof anyway. The bivy will handle spray and light splatter (fnarr!) so is handy for use under a tarp if it's breezy just to catch any stray moisture.
With Borah now seeming to have a European distributor I'm not sure if John will do direct sales to the UK. Custom stuff is likely but not the standard models. I paid $210 for the two ultralight bivvies. That included shipping but not the £8 Royal Mail release fee. This was Jan 2018. From ordering/paying it was three weeks to him shipping it.
Back to tents
I agree with Stu - you want a tent that you can at least sit up in. For most of us that's going to mean a tent with at least 90cm internal height. To me that's a problem with most of the very light tents - they just don't have a lot of space. It's all well and good saying "it weighs XXXg" but if you have to have an advanced diploma in yoga to put your socks on then it's not very usable.
Steve, it sounds to me like you are looking at spending around £500. In that situation my choices would be: the ZPacks Pleximid; MSR Hubba NX; with the Lunar Solo as a budget option. Not surprisingly with those three as the cost comes down the weight goes up.