Lawyers LOVE the K-3, added paperwork on the front end I-129F for K-3, and possible added paperwork later Adjustment of status fro K-3 to get green-card.
Consider CR-1 visa, compare time lines, you will find that for UK Cr-1 is not much longer to get.
Use timelines, filter for UK and sort by interview date, and look at the total time column.
http://www.visajourney.com/timeline/k1list...om&dfile=No (K-3)
http://www.visajourney.com/timeline/k1list...om&dfile=No (CR-1)
CR-1 has less paperwork to file over all, cost less, and results in a green-card upon entry to the USA.
Dose your future spouse want to take a JOB soon after arriving in the USA? K-3 is NOT work auth and cannot work until getting Green-card or EAD. EAD can take 90 days or more to get, all that time the K-3 holder cannot Work, get SSN or even apply for a driver's license. All these things can be done by a CR-1 visa holder because they get green-card at the time they arrive in the USA.
A Big NOTE: Lawyers cannot make the process go any faster. The community on VJ is mostly a DIY community and do not use lawyers, they just follow the guides and have no problems . Only if there is something complex about the case do people consult a lawyer.