There isn't really a cost effective way for this sort of sum. Best way (if you trust the recipient) is to buy � notes at a BdeC and send them recorded delivery mail. It will cost you �4 or so on top of the normal postage for the recorded fee and insurance.
Otherwise it's likely to be �10 to �20 for a bank transfer or cheque in ��
If the recipient has a Paypal account that's another possibility