Asian food supplies
Supermarkets are offering more choice in Asian ingredients. Large Tesco and Sainsbury's stores and most Waitrose stores offer a decent array. Morrisons, Asda and the rest will offer you more limited selections of spices, veg, rice and noodles.
The most popular products found in smaller supermarkets (Sharwoods egg noodles, sachets of sauces from companies like Amoy) don't bear much relation to Asian food as it's made in Asia. They belong to a kind of strange Anglo-Cantonese 1980s takeaway fusion that depends a lot on sugar and processed ingredients. Personally, I find them pretty sickly and heavy but each to their own.
Your best bet is to search out a decent Asian/Chinese supermarket in your nearest city or to order online. Asian ingredients generally cost less if bought for Asian stores. When I was test cooking for the book I bought all the seasoning ingredients, sauces, oils, vinegars and dried goods for all the recipes for £17 from my local Chinese supermarket. The biggest hassle in all of this is sourcing the right stuff. Having done so, the cooking itself is pretty simple and the meals are not expensive.
Chinese Food Stores in major cities
Most Chinese supermarkets will stock popular Vietnamese, Thai and Malaysian ingredients. This is not an exhaustive list. Think of it as a starting point.
Asia Supermarket 189 Ormeau Road, BT7 1SQ
Lee Foods 98-108 Donegall Pass, BT7 1BX
China Supermarket 32-34 Tudor Street, CF11 6AH
Man Fung Store 113 Penarth Road, CF11 6JT
Huaxing 48-50 Ratcliff Terrace EH9 1ST
Pat's Chung Ying 199 - 201, Leith Walk, EH6 8NX
Jing Peng 4 Wavertree Rd, L7 1PQ
Sida 192 London Road, L3
Hoo Hing Enfield (also have branches in Leyton, Mitcham and Park Royal) Lockfield Avenue Off Mollison Avenue, Brimsdown, EN3 7QE
Loon Fung Chinatown (also has branches in Tottenham, Alperton and Silvertown) 42-44 Gerrard Street, W1D 5QG
See Woo Chinatown (also has a store in Greenwich) 18-20 Lisle Street, WC2H 7BE
Wing Yip Croydon (also has a store in Cricklewood) 544 Purley Way, CR0 4NZ