I have two ideas, one is a white globe lamp, the other is a modern track system that will help you high light the art and fireplace. either will work. the ones i pictured are from Ikea, but im not sure if you have access to them. or a lighting showroom?

My light casts it's frame shadow, but it's not that noticeable because it's way up high (see pic) mine is in a double floor height ceiling. I think if the pendant was lower, then shadows would be more prominent. Also, my light isn't as busy, so the shadows are less. Overall, my take is the busier the pendant and lower the height, you'll see more shadows. (don't mind the paint job, still in reno)

I think you're right to change your plans tbh, I think pic 1 and 3 would work. Another option is a suspended woodburner. It's obviously not a divide but it gives you the break in the ceiling so acts as one. The first one attached is from fireplaceproductsUK. The other 2 are just images from leotrippi and homedesignht to show how they can act to divide a room. A much more expensive option is a glass fireplace, this is from acucraft fireplaces. I'm not sure how easy it is to find UK manufacturers/suppliers though. Alternatively you can just break the room with furniture layout and ceiling lights. The lights if hung properly ie at differing heights, clusters etc along with floor lamps will stop that cavernous feel. I can't obviously see the plan but if you're really set on a divide, and they are great!, you could set it off a side wall rather than down the centre of the room.

There's an Interesting debate about this on here http://www.electriciansforums.co.uk/electrical-forum-general-electrical-forum/63900-can-i-use-usa-light-fittings.html Best talk with your local electrician I think !!!