£5 really? Frankly I don't know exactly, but the HF2 is almost the same thing as the ER-4P, which retails for $300 and is very popular. The difference is the design, the mic, and that the HF2 has not so many checking on the production process, so some units could be "not perfect".
By the way, if you're looking for a good isolation, I think the specified 98% blocking is not exaggerated! That has it's downside of course (not knowing people are talking or shouting at you), but if you like isolation (like I do), this triple flange design of the HF2 surely is great! I'm not sure about the other in-ear buds listed here though, I have only experience with different Sennheiser and Sony models. But they all seam the same to me, so I think Etymotic has an advantage here, personally...