I'd say this time around if you want one you might as well get it now.
This time last year, it was different as it was obvious the 2G iPhone could be upgraded substantially (2G -> 3G, GPS, proper headphone jack etc).
Yes, it's likely there'll be a new iPhone announced in June but I really doubt it'll be as much of an upgrade as 2G -> 3G was. If you think about it the likely changes seem to be:
- 3.6 Mbps -> 7.2 Mbps HSDPA (Will probably make little difference on o2's Network)
- Wireless N (Little real world difference)
- Longer battery life (If you can charge daily then 3G's battery life is sufficient)
- Better camera (It's never going to be digital camera quality)
- Larger capacity (You'll have to weigh this one up for yourself, if 8/16Gb is sufficient you have no need)
Of those, none will really tempt me to upgrading. Of course apple may well surprise us with some revolutionary stuff but I'm not sure they'd bother in the current market.