I am not quite sure about this, so you would have to check it out, but I am pretty sure you can just take your receipt to the customs at Heathrow (or wherever else you are travelling from). They would want to see the computer working as well.
They fill in the papers, and give you tha VAT.
As I said, check this out with HMCustoms before travelling.
John Lewis is a good place for mac. Plus PCworld are now stocking macs.
Good luck!