I've not tried the Monarch 12x42 but I know, like many here, that I couldn't hold a small light 12x steady and would still have some difficulties with a 10x. I found the chromatic aberration fairly high on the lower powers and would expect it to be worse at 12x.
I do use a 12x occasionally for lakes and shores but it's twice the weight of the Monarchs and has fewer bad habits. I personally think the 8x42 is the choice of the range MkIII range. Perhaps you could tell us a little more about how, where and when you use your binos and we might be able to suggest some alternatives for your budget.
I wouldn't suggest 12x for general birding; in addition to the increased image shake, you will start getting into a really narrow field of view.
But if you are comfortable with 10x42 then you can stick with that. For 300 pounds you can get something a lot better then a Nikon Monarch. Since you're in the UK check out the Opticron Countryman BGA HD, or some of the ED glass from Hawke. That should be a nice starting point.
Eitan's come up with the two that I'd have suggested. The Hawke Frontier ED is a little bigger and heavier than some but with ED glass, a fairly wide flattish view it's a very popular choice. I like the Countryman HD. A little more expensive, Japanese made with fewer bells and whistles. Very much a matter of personal preference, so something to try for yourself.
Sherwoods have a good list of ex-demo Opticrons at the moment. That DBA Oasis 10x42 at £330 looks very attractive, the Imagic BGA SE 10x42 is well within budget, and personally I'd give the Countryman HD 8x42 a look. They are not too far from Birmingham. Might be worth an outing? http://www.sherwoods-photo.com/ex_demo/ex_demo_optics.htm
If you check in the review section Leif has a review of the Frontier ED. You might like to look at my report on the Eden XP 10x42 and a couple of others on the 8x42. At £245 it's a little cheaper than the Hawke and Opticron.