Agree with the reviews so far, this is the Spice store.
I live in East London and many stores have all the spices one needs to cook indian. in china town there are stores where to buy chinesse and thai, but in this very store in Portobelo there are spices, herbs, pastes and curries from all over the world.
Another point is the staff. Try to ask something in a Thai store. they may well grunt at you in a this-food-is-not-for-you fashion. Conversely, you will be advised about anything in Spice Shop. I have been just once, but I really need to come back soon to stock some stuff i would not find anywhere else.
I could not say is cheap though, but, i guess when someone is determined to try new things and get hands-on to do sexy food a few quid is irrelevant.

Just walking in here is a treat! The spice shop stocks – surprisingly – spices, from all over the world. I was completely overwhelmed by their selections. They don’t just have basic spices and herbs, they also have spice concoctions for particular recipes, or flavours, and every little bag of spice comes with instructions for cooking on the back. It’s extremely inspirational! The staff are always so helpful as well. Might be a good idea to pop into Books for Cooks across the street to get some recipes, and then go to the Spice Shop who will no doubt have everything you need.

This is THE spice shop in London. The owner does not only trade in spices, she mixes and blends and provides her own recipes. The shop located off Portobelo Road is tiny but even if you're not really into spices it's worth paying a visit just to have a look at the range of what's available. Simply amazing.