All you need is some fine gold bead chain. Any jeweller will know exactly what you're talking about and they can stamp/engrave your letter onto a disc for you. You could even change the shape of the 'disc' or the font if you please.
It would also be made to measure which means that you don't worry about having to add/take away links from the chain and you aren't buying any more than you need. (Which given the crazy high gold prices recently would be wise!)
Depending on the width of the 'beads' a 16-18 inch necklace in a solid 14k yellow gold bead chain would start at about £80. You need much less chain, only about 7 inches average for a bracelet. Plus your charm and the price for the work. I reckon you could get the whole thing done for under £150.