It's one shilling and sixpence, or eighteen pence altogether. I see that it is a quotation from "Brat Farrar" by Josephine Tey, which was published in 1949 and set in the 1940s. According to http://www.whatsthecost.com/cpi.aspx , £1 in 1949 had the same buying power as £26.5 has today. Since 18 pence was 18/240 = 0.075 pounds, 18 pence then would have had about the same buying power as £2 today. Google tells me £2 is about 19.24 yuan. It's sort of a joke, nobody would think you could buy a real diamond for "1/6" (one-and-six). Woolworth's was a cheap department store, where you would buy little things like toys or stationary or hairpins. It wouldn't even be good "costume jewelry."

Before the UK adopted decimal currency (100 pence = one pound) they used the old imperial coinage with pounds, shillings, and pennies. One and sixpence means one shilling and six pennies.

This refers to the old British currency, which was discontinued in 1971, when it went decimal. Prior to Decimal Day, there were 20 shillings in a pound (£) and 12 pence in a shilling. One shilling and six pence meant 18 old pence. There are 100 new pence in the decimal pound. 18 old pence would be equivalent to 18/240 = £0.075. In which historical period did the dialogue take place?