I don't think there's any "right" answer here, but I'd probably suggest using "right now" to get the ZoneInterval, and then offer users the choice of either entering the
standard offset (so I'd always put in UTC+0 for the UK) or the
wall offset (so in the winter I'd put in UTC+0, and in the summer I'd put in UTC+1). I can see those both being useful at different times.
In terms of how the offset is specified, I suspect that actually a textbox is the simplest approach - just indicate to the user when the input is invalid. I'd expect it to cope with:
0
+0
-0
5
-5
5:30
-5:30
If it can do all of those, you're probably fine.