My US fiancée is almost ready to file the I-129F petition, but we are currently holding back a little as I would like to visit her, and it appears that I won't be able to do this once the petition is filed - although the information on this is inconsistent at best, and at worst completely contradictory (any clarification would be gratefully accepted).

Amid all this uncertainty we have decide to play it safe for now rather than risk jeopardising the application, or buying a plane ticket only to be refused entry, and are instead planning a visit before sending off the forms. However this of course means a delay that we don't really want, so I'm wondering if it would be possible to file the I-129F while I am visiting her in the US. (I am a UK citizen.)

You are not legally prohibited from visiting the US just because a petition has been filed on your behalf.
Actually success at entry depends on a whole host of personal, individual factors.

It doesn't matter if you are in the US when she mails her forms in.

Yes, the I-129F can be filed while you are in the USA visiting.

You are allowed to visit during the I-129F and subsequent K-1 processing, but of course you are right that you could be denied entry. But simply being in the process doesn't prohibit you from visiting.

Rene

As has already been said, there's always a risk of being denied entry into the US - but that goes for everyone! I visited the US once while we were doing the fiancée visa, about halfway through the process when the I-129F had been approved, and there was no problem at all. Get the forms sent in as soon as you can, don't delay filing just because you want to visit the US. It's a good idea if you're somewhat of a 'frequent' visitor to the US, or 'high risk of staying' (as the fiancé of a US citizen), to take proof of ties to the UK with you on your trip, just in case they ask for it (they never did with me, but it gave me peace of mind knowing that I had it if they requested it).

Good luck with your visa process!