Prices in 1978's England?

[beeinmybonnet]

Hello, all you sages! ♥

I'm writing an AU fanfic which takes place in London during 1978 (around autumn, if that's any help). Now, I'm neurotic when it comes to details -- and this is actually sort of important to the plot -- I need to know what a few, very specific things cost back then. Unfortunately I don't know anyone who lived in England during that time, and I've tried various searches using Google, but to no avail. Maybe you can help me?

What I need to know the price of are these things:

• A plectrum for guitars -- a guitar pick, if you like.
• A carton of milk.
• A packet of spaghetti.

Now that I look at it, it looks really ridiculous, but I don't know how to get the information otherwise. One of the sub-plots is that character A buys character B these things, B won't have charity and tries to pay A back, which A won't accept that and they keep arguing about this.
Ought to teach me to just write things set during my own time period. :P