Well I use a Kathmandu basha at 2.5m square and a Rab survival zone. All in it is lighter than all but the Laser Photon (or is it? mine is about 650g the photon is760g isn't it?) and the new TN Laser with the cuben fibre outer. Those are also too short for me to fit in as I am 6'5" tall. Anyway the tarp and lightweight bivvy is better. Can I suggest that the ex surplus bivvies are too heavy and if you go that way then a 1kg tent would be better. I havew no choice as I can not afford a UL tent that is long enough and they are too heavy anyway.
IMHO get yourself a lighter bivvy such as the Rab SZ or the Alpkit hunka (also available in XL which is the same length as the Rab SZ). Or get yourself onto the Mountain Laurel Designs website and see what they can offer. American company and highly regarded. Probably more than you want to spend though but you want the best then might have to pay for it. Check out BPL-UK for their UL mini tarp thingy
http://www.backpackinglight.co.uk/product157.asp?PageID=112 or for other stuff
http://www.backpackinglight.co.uk/page112.asp
The 148g micro tarp covers your upper body and TBH that will make a big difference over using your bivvy on its own. I know from threads on another forum which the owner of BPL-UK posts on he has used the Rab survival zone (SZ) bivvy with their micro tarp for years. He puts the bivvy directly on the groung too. I have only done that once but tend to carry a free UL groundsheet I got from work (pallet bag cut up to as small a piece as would work for me). I would always use a lightweight bivvy with my tarp even though it can form tent like shelter. I use mine, because it is big, in a cave arrangement so it is only open on one side. This side can be lifted or lowered as the conditions dictate. You can even get dodgers for the front IF the weather was going to be so bad you needed a door.
http://www.granitegearstore.com/Dodger-P99C27.aspx
The last link is from their clearance page so I think they are stopping making them. These are crying out for peole tomake themselves. MYOG is a growing thing although not for me it might be a good way for you to get a tarp/bivvy kit that suits you. Perhaps make your own dodgers for a tarp pitched either as an a frame or a cave type. Like with everything to do with outdoors you have to get the right kit for what you want/intend to do. BPL-UK offer some good and cost effective solutions to shelter and other wildcamping things. I hope this helps. Personally I would stay away from a lot of surplus or ex issue kit not because it won't do the job but because it of the fact it will be made squaddie proof so will be heavier than the civilian designed and made stuff.
One more thing, with a larger tarp (say 2.4m long by say about 1.2m wide or bigger) you could get away with a sleeping bag cover. These are not waterproof but water resistant. They are designed to cope with dripping on down sleeping bags from snowholes and the like but not direct rain. They are a lot lighter too. They do cut out the wind which could get under the tarp. The Rab SZ is almost as light but IS waterproof and breathable (never got damp inside due to overcoming the breathability or leakage so far touch wood and dance around in a superstitious way). Have fun, tarp and bivvying on the ground is fun. PLanning my first ever winter camp in the Lakes to be a tarp and bivvy camp early next year. Thinking of doing it anyway, probably chicken out.