Were can you bye osymetric chainrings in the uk and do you need to have your gearing adjusted
no idea about gearing im afraid.
...You could drop the places selling them an email though.
Any decent LBS can order them in. I got a set on my last bike purchase. The front dérailleur will probably need its height adjusted to accommodate the larger width of the ring at its widest point if upgrading an existing bike.
If it hadn't been part of a new bike build not sure I would have bothered, I can't honestly say I've noticed any difference!