Don't post cash. The letter could go astray or be destroyed or damaged for many many reasons in transit, and though unlikely, it COULD happen. It isn't worth it.
To be honest, I'd distrust the recipient (if unknown to me) more than the mail if it was someone insisting on cash payment, for example. I wouldn't send that sort of sum in the normal mail anyway.
Special Delivery will cost �4.30 upwards, depending on how much insurance you want. I <think> the basic fee covers up to �500.
Above �50 (roughly - can't remember the exact amount but 8.75% as postal order fees sticks in my mind) ) a postal order will actually cost more
I have in the recent past, sent notes in the ordinary post, sometimes alone in the envelope and another time inside a card. Always arrived without any problem. I know I should not have done this but just to say I did and it got there ok.