Please think carefully about the visit. We are on round 2 of our 129F, my fiance is also from the UK. Our lawyer who is helping us this time around advised us NOT to have him visit because chances are he might not be permitted entry into the US based upon the 129f being in process. We are meeting up instead in Toronto for 3 weeks. I'll drive up, he'll fly in. We didn't want to take the chance that he would spend the money on the ticket only to be turned back at the borders.
Hi, OK... so I really wish they would compile a thread about visiting while the K1 is in process because of all the conflicting advice. I don't know why your lawyer told you that, because the one I consulted told me the exact opposite. Also, I called customs passenger services at the airport and they said there is ABSOLUTELY NO PROBLEM with visiting while the K1 is in process. Now, this may be different for people from countries that don't fall under the visa waiver program but for customs, the biggest issue is whether the person traveling qualifies to visit under the VWP. Meaning, they have to have a return flight, visit for legitimate tourist purposes, etc. This is directly from customs themselves. Any lawyer that advises you otherwise is incorrect and doesn't understand customs procedures.
To the OP - only listen to people who are educated about this topic and have personal experience to give because I am doing the exact same thing you are. My guy is from england and had no problems visiting after we filed his I-129F. We filed in December and expected to get the NOA2 in May. He arrived here at the beginning of February for 89 days under the visa waiver program. We were worried, but had no problems at customs. My guy was up front at customs and said he was visiting his fiancee. When the customs agent asked why he was staying so long, he said, "To meet her family more and get to know them, to see the sights of the city she lives in, I have a return flight, would you like to see it?" She said yes, and after he showed the return flight, she said OK and let him in without any secondary screening.
Yes, we were worried, but if the traveler has a return flight and any ties to home, that's a good thing. The lawyer I called even said, "with a visa waiver country, it's a lot easier" The only issue would be if there are repeated trips for 89 days.
What we did... he was here for 89 days and we got the NOA2 on the day after he left. I really couldn't have guessed that part yet. Well, the timeline that you're seeing is only for the I-129F, so those estimations are when you'll get the NOA2. After the NOA2 it goes a lot faster, but for the UK at least, it'll probably be about 3 months from the NOA2 date to the interview date. Check out my timeline right now, it's been a little less than a month since I got my NOA2 and we have the medical exam booked. It'll probably be a month from then until we get our interview date...
So, I think it's OK that you visit under the VWP for less than 90 days, the beneficiary should still have a place to stay back home for 3 months while they await the medical exam/interview if you plan the visit out to wait while the NOA2 comes. I think it's a great idea what you're doing because it made the NOA2 wait go a lot faster

Best of luck to you!!!