There are no good options.
BMW V8s are fine, but the manual boxes are V8 specific because the v8 sits upright and the I6 inclined so the bellhousing is aligned differently. Good luck finding a V8 box. Adaptors to the common and cheap 6 cyl boxes are available, but then you need a flywheel and clutch to replace the flex plate.
http://www.ebay.co.uk/itm/BMW-V8-M6...D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557
Audi V8s are also great, and audi manual boxes fit, which is great if you want a longitudinal mid rear layout, and useless for anything else.
Rover V8s dont make the power and the box of choice is a T5 and the price of those has gone through the roof.
Jag made some decent engines, but sadly all auto only.
Mercedes is the same story
The Lexus 1UZFE is a decent strong engine for buttons, but no manual option. People use the supra box with an off the shelf adaptor, but as a result, the cost of the boxes has gone through the roof.
American stuff is fine, but whereas a 5.3 vortec is ~$1000 and an 6.0 LS is ~$3000 over there, by the time you get them here, thats £3500 and £4000 so you may as well have the larger alloy unit, and then you want to find a T56 box and so on. You wont have it in the car for less than 5k. The older stuff like the SBC is also great and rebuild parts are cheap, but the current exchange rate and import duty means its actually not.