It depends on the size, weight, destination, type of service and value. Go to www.royalmail.com, where you can put in the relevant details and get a price.
Is it smaller than 240mm x 165mm (9.5ins x 6.5 ins) and less than 5mm thick? Tha's a letter. Bigger, or thicker, makes it a large letter or a packet. What is the value? How urgent is it?
If you're sending it special delivery then it goes on weight, not whether it is letter, large letter or packet sized. Size only matters when it gets much bigger. But then you could send it Parcelforce. Of course, the best answer is to take it to your local Post Office!