Which is the most reliable Royal Mail service?

I have some important documents to send through mail with in the UK. Time is not a problem and I would not mind even if it takes 3-5 days to reach the destination but I want the mail to be traceable or something like which requires signature from the person receiving the letter. Can anybody help please?Which is the most reliable Royal Mail service?
special delivery.it is traceable and comes with a guaranteed delivery date.it is also signed for on arrival.you check the item progress on-line or by telephone the next working day.it can be guaranteed to be delivered before 9am or before 1pm.

Kate recorded delivery is not the most reliable way to post.your item still travels 1st/2nd class mail and if lost you cannot submit a claim for 15 days if lost or delayed.Special delivery can also insure you item for up to 拢2500 and costs only 拢5-05 by far cheaper than any private carrier.Which is the most reliable Royal Mail service?
Recorded Delivery (or it might be called Registered Post now). Just go into the post office and ask for your letter to be sent registered post - it doesn't cost that much and it basically means the letter has to be signed for by the recipient. Make sure they don't try to send it Special delivery, that costs a bomb and isn't necessary!Which is the most reliable Royal Mail service?
Recorded delivery is best. I used this when I served an eviction notice on tenants and they had to sign to receive the letter so I had proof they got it.