I highly recommend paying the extra for the automatic, even if you know how to drive a stick (with your right hand). Shifting with your left adds yet another layer of disorientation to your driving experience, and since it's potentially a personal safety issue and as well as a stress reducing issue, go ahead and treat yourself to the automatic.
This just isn't to make your experience more pleasant, it's potentially a personal safety and financial issue.
I take my own advice when renting in UK, and drive a stick everywhere else in Europe.
But don't just check the automatic box in the online form and think that takes care of it in the UK. It doesn't. If you really want an automatic, I suggest you do what I've done and telephone the UK office about two days before your departure, reference your contract # so they're looking at your info, and tell them "I really, really want an automatic." That should do it, at least it's worked for me in situations where I realized later that if I hadn't done it, I would have gotten to the rental lot and been told, "sorry sir, we're out of automatics."
It's different over there.