How does Bitcoin Gigahashes Convert to Ethereum Megahashes?
Bitcoin and Ethereum use different hashing algorithms. General consumer hardware (like a CPU or GPU) can be easily programmed at a software level to perform either the Bitcoin or the Ethereum (amongst many other) hashing algorithms. However, specialized hardware (like the Block Erupters you mentioned, which are Application Specific Integrated Circuits (ASICs)) have the hashing algorithm hard-coded into the chip's design, allowing it to much more efficiently perform one type of work (Bitcoin's double-SHA256, for example) at the expense of flexibility--they can't be reprogrammed to do anything else.

On a side-note, a £60 ASIC which runs at 16 GH/s (which isn't a block erupter, it might be the USB Stick from GekkoScience?) is very unlikely to ever reach ROI. At current difficulty and Bitcoin price, and assuming free electricity, a 16 GH/s miner would bring in about £0.26/month, which is likely to get even worse as more mining power comes online in the future.