I have all your cook books. The only thing that confuses is your being in the UK. I do not know what Guinness is. I do not know Marmite or vegmite is. I have never seen or tasted a Quince in the United States. So please tell me, what is Guinness? As far as I know, the other stuff I mentioned are not sold or known in the U.S.
Guinness is a type of dark ale known as stout. It is made with barley that has been roasted for longer than for normal ales to give a slightly bittersweet taste and a very dark color. Guinness is a very famous irish brand and is widely distributed in the US and throughout the world. The web link for Guinness in the US is below but is should be available in some supermarkets and liquor stores.
Marmite and Vegemite are both thick spreads with a very salty and savoury (umami-type) taste. They can be spread sparingly on buttered toast or bread and can also be used in some recipes. They are both made from yeast extract which is a by-product from the brewing process, but they don't contain any alcohol. Marmite is available in the US in some gourmet stores and also on-line from British delights (in the condiments section).
Quinces are a fruit which looks a little like a pear (it is related to apples and pears) but it is very hard and slightly sour and needs cooking before eating. Its flesh changes from white to pinky-red when cooked. However the main attraction of a quince is its wonderful aroma and a bowl of quinces will perfume a room very quickly. Quinces are high in pectin so are very good for jams and jellies and are also popular in some savoury dishes in the Middle East. They have a fairly short season and can be difficult to find both in the Uk and the US. We would suggest speaking to your local gourmet store or checking farmers markets in the fall to see if you can find them.