Doesn't look unreasonable to d-lock the bike to those railings, I'd get a couple of d-locks, they don't have to be expensive, they just have to
look expensive. For my bike I use a Kryptonite Evolution Mini 5 lock (about £50 when on offer) which I put through the frame - it's big enough to lock through the bike and most bike stands with a little wiggle room, but not so big that it leaves lots of space to get a tool through.
A very good tip for using a d-lock in spaces like that is to fit it so the lock component is on the "wrong" side of the railings, so that anyone who doesn't have the key needs to access it from the stair side, which makes it inconvenient at best. I'd use a good lock like that or an Abus, and add a second d-lock at a later date, which need not be so expensive. The more locks the better.
The objective here is to make it enough of a hassle that stealing somebody else's bike is more appealing.
Edit to add:
Sigma Sports have the
Kryptonite Mini 5 on offer at £44 now, which I think is a very good deal.