For what it is worth, I don't generally recommend leaning mirrors over fireplaces. They may reflect a chandelier, but what they primarily reflect is the ceiling. Not attractive in most instances. Just food for thought... The above recommendations are both terrific. Be sure they are scale appropriate.

I think the large mirror covering the whole wall will be too big with the tv right there. I am into the antlers but I don't think they relate to anything else in the room other than the fireplace itself. I think your best bet is a simple round mirror. I know you've been shopping locally - if you are open to online, overstock has a big selection. Here is one that incorporates some gold and silver so it picks up your chandelier but is still pretty simple. http://www.overstock.com/Home-Garden/Raymond-Round-Mirror/8403155/product.html?refccid=2CDPCEBNTZLJLASNJPTJVEOR7M&searchidx=14

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.

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