Unless I'm mistaken, this seems to be the lowest price currently for a new copy of Sonic Generations 3DS. I just reserved one for tomorrow so if anyone else is interested, take a look because despite being out of stock at most, there's a few stores that still might have it.
Amazon appears to be the next best choice afterwards who're selling it for £17.75 but it's on backorder. I tried Flubit but it'd always come up with £17ish prices.
The specifically designed Nintendo 3DS environments, based on iconic levels from popular Sonic the Hedgehog titles, will include an exclusive unlockable ‘Special Stage’ allowing players to become even more immersed in Sonic’s world. Sonic Generations for Nintendo 3DS will also take advantage of the StreetPass feature by enabling players to collect new content and unlock bonus material each time they pass another player. In addition to this, the new two-player versus mode means fans can battle it out across some of the most recognisable Sonic environments in a completely new way.
TCB cashback also available as well as Quidco as pointed out by TheVMan2k3