This is a literal 'metal Sonic', figure, that isn't of actual Metal Sonic. It just happens to be made of a pewter tone metal of some kind. This little figure is from SegaWorld England. What is it for? Looks like nothing but display. It's an ok little figure of Sonic, but his eye-ridge is a bit excessive. This figurine is not common. It is quite small.

A Rare Item!
This is a pewter figure from Segaworld London. It's classic Sonic, in a walking/running forward pose. He's on a little base that spells out "Segaworld" in the 'Sega' Font. It's raised/textured on the circle shaped base. As you can see with the hand in the photo, it's not a very big figure at all.
Why is it uncommon? Because Segaworld London didn't last all that long as an attraction / it's not still around like the ones in Japan.

Also, it's a small, gray figure that doesn't do anything so it's not going to attract tons of little kids into buying it. Then, it was likely at least a little costly upon release because while Pewter isn't a precious metal, it's not a common/base one either. (Semiprecious metal?) Next, it's small so if someone is going to lose something, this could well be it. (Pewter's also pretty soft so who knows if someone ever bent theirs over the years)
The likeness is ok, especially considering the time it was made. It was barely known what Sonic would look like from the back at that time. The eyes are a little big, but it is still pretty Sonic-like and good to collect...if you can find one.