Specifically on the processor it may be cheaper to get the base and replace it immediately for £1800 [1] than to buy the top of the line than to pay Apple the £2800 extra it seems to ask for what appears to be the same processor (cache, speed at least). And you might be able to sell the original.

[1] http://www.scan.co.uk/products/intel-xeon-e5-2697-v2-s-2011-...