You did not mention the exact spec of the PC but according to your maths that means a top of the line iMac costs £2891!
27" High end model (delivered including tax) with no BTO costs £1699.
If you upgrade the CPU to 3.9Ghz and the graphics to the 680 2GB card it costs £1,978.99. If you add the SSD "Fusion drive" option it will jump by £200.
Nobody ever buys RAM from Apple as you can just buy that from crucial for half the price so I am not including that.
This gives a total of £2,179.00 for the following spec.
3.4GHz Quad-core Intel Core i7, Turbo Boost up to 3.9GHz
8GB 1600MHz DDR3 SDRAM - 2x4GB
1TB Fusion Drive
NVIDIA GeForce GTX 680MX 2GB GDDR5
I know it's not "maxed out" compared to all the options but the only way you can get the number you quoted is by maxing out things that don't really matter which I don't think is really a fair comparison. The PC is still has better hardware in part (I don't know about RAM or CPU etc) but the iMac is suddenly not looking as bad value.
For the £3000 pound price you mentioned you are better placed investing in a new MacPro which although more price should blow the PC you mentioned out the water with 12 cores and 6GB of VRAM using SLI AMD fire cards.
Yes I know that is not fair either but that's kinda my point if you want you can make the Mac look like poor valve or good value depending on how you spec up the machine and what you want it to do. The Mac will never be a "hardcore gamer" machine but neither is it poor value.
Edwin