I found this page super helpful, so in case it helps any others thought I'd post my experience getting this done in London.
- the Omega boutiques in London will sell you an 1125/617, but the clerks/salespeople are generally clueless, i went to two boutiques and both had to be walked through it in painstaking detail (at first they'll say the watch doesn't come with a steel bracelet and stare blankly at you, but if you just keep repeating as many times as necessary (4 or 5 times, then once more when they call over the manager) that you know that, but you want an 1125/617 please, and can they please just order it for you, you'll get there in the end.
- cost was £335 (Q4 2018).
- the first boutique said 2-3 months to arrive. the second (who i ordered from), said 2 to 3 weeks, and it ultimately arrived after about 5 weeks.
- not sure about any of the filing on the 617 discussion, i had the boutique fit it for me (they did it for free), and they didn't mention anything about that (i also insisted they put new, size appropriate spring bars in, which they did, and also didn't charge for, which was nice).
- i've since flipped it back and forth between leather strap and the 1125/617, and the changeover is generally pretty easy (i use a pretty decent Bergeon spring-bar tool to do it each time). not as easy as changing a Rolex strap, but definitely doable by an amateur (like me).
- overall, super pleased with the look of the steel strap. very glad it got this done (and that this page exists to put me onto it in the first place!)