The truth is no one knows. Right now, CR1s are faster, however, USCIS is hiring quickly and we should see backlogs easing over the next year. Most embassies are improving their backlog, and I don't believe the UK has significant waits. Our NOA2 took 16 months, well past the 5-7 months quoted when we started. Had we know the actual duration, we would have made very different decisions (such as staying in Italy together for several more months). Unfortunately, we were at 11 months before USCIS actually started correcting the estimated wait time. Fortunately, you and your F will be able to visit via ESTA. Many couples from other countries cannot easily. We were fortunate to be able to visit here in the US 3 times for a month or more at a time.
The last year was very hard on our family, and unfortunately, you cannot do anything to speed up the process with K1 or CR1. So, knowing the long list of possible unknowns, make the decision that is best for you and your F. I wish you both the very best!