11Mbs is 802.11b and in real world terms about 6Mbs in a reasonable range.
54Mbs is 802.11g and is better but still not perfect.
Both are very useful for specific tasks, neither is secure. If you want security get wires, there is no substitute for security and performance. Basically the difference is in the speed that the wireless protocal transmits the data, the more Mbs the faster it is.
As for broadband providers, look at http://www.adslguide.org.uk for the layest and greatest, but either Zen or Eclipse seem to be the best choice right now.
K.