If doing a return same day, and have a railcard, then it works out at only £1.85 each way (£3.70) by getting a CDR from Finsbury Pk to Clapham Jn, this has the cross-London marker and will work the barriers between any pair of LU London Terminal stations. This isn't much more than an Oyster, but it is easy to buy in advance and does not threaten to charge you loads of money if you touch in at the wrong Oyster reader, or the system accuses you of failing to touch out. I've heard several stories of people being overcharged on Oyster, so it is safer to avoid using it if possible. I do have an Oyster card but I rarely use it as I'm usually on either a Travelcard or a ticket that includes cross-London transfer.