Looking for some advice on the next step with my '03 320i

It's burning oil at a horrendous rate - best guess is around 2L per 700miles.
It's burning , not leaking, evidenced by the smoke on acceleration at low speed - no trail of blue smoke while driving, valve stem seal test consistently gives me the blue plume.
I have it taxed till Oct and NCT'd till april so there is some time to make a move yet, my question is to those that have experienced this , should I go to the trouble/expense of having the valve stem/PCV/oil rings replaced ?What sort of money would I be looking at here?
Is there any alternative to reduce oil consumption? I have started to use a 10/40 and that has helped....marginally.

I would appreciate some guidance from experience.