It's the Distance Selling Regulations, so it only applies to mail-order. They are required to replace or reimburse you to the state you were in before the transaction. You are not required to pay postage and only required to have the goods available at the location they were delivered to, although I dare say a free or refunded postage would be reasonable. It would be the whole kit though.
The clause in the DSR is the one obtaining when the seller is unable to comply with the contract, which here is to provide you with a non-faulty keyboard.
The Office of Fair Trading says this:
Only if it is covered in the contract and the written information
can you require the consumer to pay for the cost of returning
the ordered goods. If the consumer then fails to return the
goods, or sends them at your expense, you can charge them
the direct cost to you of the return, even if you have already
refunded the consumer’s money. You are not allowed to
make any further charges, such as a restocking charge or an
If you do not include these details in the required written
information then you cannot charge anything.