Timpsons can do you a new key with transponder but they are very basic cheapo keys.
Consider carkeyhelp.co.uk for a cut key from a good digital photo; and what I did was order the plastic wallet key from bmminiparts special price finder for about £30 (after proof of ownership supplied) which has a black sticker on the back which revealed a transponder to transfer to the newly cut key.
EDIT: carkeyhelp.co.uk BMW keys look a bit gash, not as good as the Toyota ones. You can get new blank replica keys from eBay and a local key place can cut them for £10-20, so long as you reassure them the transponder doesn't need doing there.
Frankly if you are selling it then save your money. It's £60 you won't get back. Most 10 year old cars only have 1 key these days. Not sure how it happens that someone looses all the keys