The Hut Group is now listing some Sonic Generations pre-order bonuses and game combo deals for the UK across all of their retailers. With the Xbox 360 version you get a set of three Avatar costumes, including Sonic, Super Sonic and Classic Amy Rose. With the PS3 version you get a Dynamic Theme featuring Classic Sonic and Modern Sonic.

You can get your pre-order(s) in at the links below:
The Hut
Xbox 360 (with bonus Avatars): £34.85
PS3 (with bonus Dynamic Theme): £34.99

Zavvi
Xbox 360 (with bonus Avatars): £34.85
PS3 (with bonus Dynamic Theme): £34.85

Woolworths Entertainment
Xbox 360 (with bonus Avatars): £34.99
PS3 (with bonus Dynamic Theme): £34.99

The combo deals available allow you to get SEGA Mega Drive Ultimate Collection (PS3 & Xbox 360), SEGA Mega Drive Classic Collection: Volume 1 (PC) and Super Monkey Ball 3D (3DS) at a discount when purchased with Sonic Generations.

You can get your pre-order(s) in at the links below:
The Hut
Xbox 360 (with SEGA Mega Drive Ultimate Collection): £34.95
PC (with SEGA Mega Drive Classic Collection: Volume 1): £14.95
3DS (with Super Monkey Ball 3D): £34.95

Zavvi
Xbox 360 (with SEGA Mega Drive Ultimate Collection): £35.85
PS3 (with SEGA Mega Drive Ultimate Collection): £35.85
PC (with SEGA Mega Drive Classic Collection: Volume 1): £17.95
3DS (with Super Monkey Ball 3D): £35.85

Woolworths Entertainment
Xbox 360 (with SEGA Mega Drive Ultimate Collection): £37.97
PC (with SEGA Mega Drive Classic Collection: Volume 1): £19.97
3DS (with Super Monkey Ball 3D): £37.97

Their help section says they deliver to all countries outside of the UK except to the below:

Russia
Brazil
Syria
North Korea
Iran
Cuba
Sudan
Myanmar
Libya

Thanks to Hogfather, 06 Inferno and MarcelloF for the heads up!